feat: Add force_hierarchical mode to prevent fallback

## Changes

Allow users to force hierarchical search mode without fallback to simple
search, enabling testing of hierarchical UI even when 0 summaries are found.

**Backend (flask_app.py):**
- Added `force_hierarchical` parameter to `hierarchical_search()`
- When True, never fallback to simple search (return empty hierarchical result)
- Added `fallback_reason` field to explain why no results
- Pass `force_hierarchical=True` when `force_mode == "hierarchical"`
- Applied to all fallback points:
  - No Weaviate client
  - No summaries found in Stage 1
  - No sections after author/work filtering
  - Exception during search

**Frontend (templates/search.html):**
- Display warning message when `fallback_reason` exists
- Yellow alert box with explanation and suggestions
- Works even when `results_data.results` is empty

## Usage

1. Select "🌳 Hiérarchique (2-étapes)" in Mode dropdown
2. Enter any query (even if no matching summaries)
3. See hierarchical UI with warning instead of fallback

## Example

Query: "Qu'est-ce que la justice ?" (not in Peirce corpus)
- Mode forced: Hierarchical
- Result: 0 sections, warning displayed
- No silent fallback to simple search

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

generations/library_rag/flask_app.py

@@ -291,6 +291,7 @@ def hierarchical_search(
     author_filter: Optional[str] = None,
     work_filter: Optional[str] = None,
     sections_limit: int = 5,
+    force_hierarchical: bool = False,
 ) -> Dict[str, Any]:
     """Two-stage hierarchical semantic search: Summary → Chunks.
 
@@ -303,6 +304,7 @@ def hierarchical_search(
         author_filter: Filter by author name.
         work_filter: Filter by work title.
         sections_limit: Number of top sections to retrieve (default: 5).
+        force_hierarchical: If True, never fallback to simple search (for testing).
 
     Returns:
         Dictionary with hierarchical search results:
@@ -310,17 +312,28 @@ def hierarchical_search(
         - sections: List of section dictionaries with nested chunks
         - results: Flat list of all chunks (for compatibility)
         - total_chunks: Total number of chunks found
+        - fallback_reason: Explanation if forced but 0 results (optional)
     """
     try:
         with get_weaviate_client() as client:
             if client is None:
-                # Fallback to simple search
+                # Fallback to simple search only if not forced
+                if not force_hierarchical:
                     results = simple_search(query, limit, author_filter, work_filter)
                     return {
                         "mode": "simple",
                         "results": results,
                         "total_chunks": len(results),
                     }
+                else:
+                    # Forced hierarchical with no client
+                    return {
+                        "mode": "hierarchical",
+                        "sections": [],
+                        "results": [],
+                        "total_chunks": 0,
+                        "fallback_reason": "Weaviate client unavailable",
+                    }
 
             # ═══════════════════════════════════════════════════════════════
             # STAGE 1: Search Summary collection for relevant sections
@@ -338,13 +351,24 @@ def hierarchical_search(
             )
 
             if not summaries_result.objects:
-                # No summaries found, fallback to simple search
+                # No summaries found
+                if not force_hierarchical:
+                    # Auto-detection: fallback to simple search
                     results = simple_search(query, limit, author_filter, work_filter)
                     return {
                         "mode": "simple",
                         "results": results,
                         "total_chunks": len(results),
                     }
+                else:
+                    # Forced hierarchical: return empty hierarchical result
+                    return {
+                        "mode": "hierarchical",
+                        "sections": [],
+                        "results": [],
+                        "total_chunks": 0,
+                        "fallback_reason": f"Aucune section pertinente trouvée (0/{sections_limit} summaries)",
+                    }
 
             # Extract section data
             sections_data = []
@@ -393,13 +417,27 @@ def hierarchical_search(
                 sections_data = filtered_sections
 
             if not sections_data:
-                # No sections match filters, fallback to simple search
+                # No sections match filters
+                if not force_hierarchical:
+                    # Auto-detection: fallback to simple search
                     results = simple_search(query, limit, author_filter, work_filter)
                     return {
                         "mode": "simple",
                         "results": results,
                         "total_chunks": len(results),
                     }
+                else:
+                    # Forced hierarchical: return empty hierarchical result
+                    filters_str = f"author={author_filter}" if author_filter else ""
+                    if work_filter:
+                        filters_str += f", work={work_filter}" if filters_str else f"work={work_filter}"
+                    return {
+                        "mode": "hierarchical",
+                        "sections": [],
+                        "results": [],
+                        "total_chunks": 0,
+                        "fallback_reason": f"Aucune section ne correspond aux filtres ({filters_str})",
+                    }
 
             # ═══════════════════════════════════════════════════════════════
             # STAGE 2: Search Chunk collection filtered by sections
@@ -461,13 +499,23 @@ def hierarchical_search(
 
     except Exception as e:
         print(f"Erreur recherche hiérarchique: {e}")
-        # Fallback to simple search on error
+        # Fallback to simple search on error (unless forced)
+        if not force_hierarchical:
             results = simple_search(query, limit, author_filter, work_filter)
             return {
                 "mode": "simple",
                 "results": results,
                 "total_chunks": len(results),
             }
+        else:
+            # Forced hierarchical: return error in hierarchical format
+            return {
+                "mode": "hierarchical",
+                "sections": [],
+                "results": [],
+                "total_chunks": 0,
+                "fallback_reason": f"Erreur lors de la recherche hiérarchique: {str(e)}",
+            }
 
 
 def should_use_hierarchical_search(query: str) -> bool:
@@ -584,6 +632,7 @@ def search_passages(
             author_filter=author_filter,
             work_filter=work_filter,
             sections_limit=sections_limit,
+            force_hierarchical=(force_mode == "hierarchical"),  # No fallback if explicitly forced
         )
     else:
         results = simple_search(query, limit, author_filter, work_filter)

generations/library_rag/templates/search.html

@@ -130,7 +130,7 @@
     {% if query %}
         <div class="ornament">·</div>
 
-        {% if results_data and results_data.results %}
+        {% if results_data %}
             <!-- Mode indicator and stats -->
             <div class="mb-3" style="display: flex; align-items: center; gap: 1rem; flex-wrap: wrap;">
                 <div>
@@ -160,6 +160,17 @@
                 {% endif %}
             </div>
 
+            <!-- Fallback reason warning (when forced hierarchical but no results) -->
+            {% if results_data.fallback_reason %}
+                <div class="alert" style="background-color: #fff3cd; border: 1px solid #ffc107; color: #856404; padding: 1rem; border-radius: 4px; margin-bottom: 1rem;">
+                    <strong>⚠️ Mode hiérarchique forcé :</strong> {{ results_data.fallback_reason }}
+                    <br>
+                    <small>💡 Essayez une requête sur un sujet présent dans le corpus (ex: "croyance", "signe", "inférence") ou basculez en mode Auto-détection.</small>
+                </div>
+            {% endif %}
+
+            {% if results_data.results %}
+
             <!-- Hierarchical display -->
             {% if results_data.mode == "hierarchical" and results_data.sections %}
                 {% for section in results_data.sections %}