Add re-analyse button and POST /documents/{id}/reprocess endpoint

Resets status to pending, clears error_message, and re-enqueues the
background AI extraction task. Button is disabled while the document
is already pending or processing; returns 409 in that case from the API.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

features/doc-service/STATUS.md

@@ -33,6 +33,7 @@ Database: shared PostgreSQL instance, isolated via `alembic_version_doc_service`
 | `PATCH` | `/documents/{id}/title` | Update editable title |
 | `GET` | `/documents/categories` | List all categories for the user |
 | `POST` | `/documents/categories` | Create a category; triggers re-analysis of documents in similar categories |
+| `POST` | `/documents/{id}/reprocess` | Reset status to pending and re-run AI extraction; 409 if already pending/processing |
 | `PATCH` | `/documents/categories/{id}` | Rename a category |
 | `DELETE` | `/documents/categories/{id}` | Delete a category |
 | `POST` | `/documents/{id}/categories/{cat_id}` | Assign category to document |
@@ -138,7 +139,7 @@ backend (proxy)  →  doc-service:8001
 
 ## Future work
 
-- [ ] `POST /documents/{id}/reprocess` — re-run AI extraction
+- [x] `POST /documents/{id}/reprocess` — re-run AI extraction
 - [ ] Advanced filter: query `extracted_data` JSON fields (vendor, due_date, amount) — requires PostgreSQL `jsonb` column or indexed virtual columns
 - [ ] Bulk operations endpoint
 - [ ] Document sharing via groups (blocked on groups/permissions system in backend)