Add unique constraint to prevent duplicate (author, chapter, page) submissions

Adds a PostgreSQL partial unique index on (author, chapter, page) where all
three fields are non-null, and returns HTTP 409 when a duplicate is detected.

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

backend/main.py

@@ -675,6 +675,12 @@ async def commit_job(
             os.remove(image_path)
         except Exception:
             pass
+        # Unique constraint violation (author + chapter + page already exists)
+        if getattr(exc, 'pgcode', None) == '23505':
+            raise HTTPException(
+                status_code=409,
+                detail="A job with this Author, Chapter, and Page already exists."
+            )
         print(f"Job commit DB error: {exc}")
         raise HTTPException(status_code=500, detail="Failed to save job to database.")