feat: Add multi-file batch upload with sequential processing

Implements comprehensive batch upload system with real-time progress tracking:

Backend Infrastructure:
- Add batch_jobs global dict for batch orchestration
- Add BatchFileInfo and BatchJob TypedDicts to utils/types.py
- Create run_batch_sequential() worker function with thread.join() synchronization
- Modify /upload POST route to detect single vs multi-file uploads
- Add 3 batch API routes: /upload/batch/progress, /status, /result
- Add timestamp_to_date Jinja2 template filter

Frontend:
- Update upload.html with 'multiple' attribute and file counter
- Create upload_batch_progress.html: Real-time dashboard with SSE per file
- Create upload_batch_result.html: Final summary with statistics

Architecture:
- Backward compatible: single-file upload unchanged
- Sequential processing: one file after another (respects API limits)
- N parallel SSE connections: one per file for real-time progress
- Polling mechanism to discover job IDs as files start processing
- 1-hour timeout per file with error handling and continuation

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

generations/library_rag/utils/types.py

@@ -1216,3 +1216,47 @@ class DeleteDocumentResult(TypedDict, total=False):
     deleted_sections: int
     deleted_document: bool
     error: Optional[str]
+
+
+class BatchFileInfo(TypedDict, total=False):
+    """Information about a single file in a batch upload.
+
+    Attributes:
+        filename: Original filename
+        job_id: Processing job ID (assigned when processing starts)
+        status: Current status (pending, processing, complete, error)
+        error: Error message if processing failed
+        size_bytes: File size in bytes
+    """
+
+    filename: str
+    job_id: Optional[str]
+    status: str  # Literal["pending", "processing", "complete", "error"]
+    error: Optional[str]
+    size_bytes: int
+
+
+class BatchJob(TypedDict, total=False):
+    """Batch processing job tracking multiple file uploads.
+
+    Attributes:
+        job_ids: List of individual processing job IDs
+        files: List of file information dictionaries
+        total_files: Total number of files in batch
+        completed_files: Number of files successfully processed
+        failed_files: Number of files that failed processing
+        status: Overall batch status (processing, complete, partial)
+        current_job_id: Currently processing job ID (None if between files)
+        options: Processing options applied to all files
+        created_at: Timestamp when batch was created
+    """
+
+    job_ids: List[str]
+    files: List[BatchFileInfo]
+    total_files: int
+    completed_files: int
+    failed_files: int
+    status: str  # Literal["processing", "complete", "partial"]
+    current_job_id: Optional[str]
+    options: ProcessingOptions
+    created_at: float