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/database.py

@@ -45,6 +45,13 @@ def init_db():
             cur.execute("""
                 CREATE INDEX IF NOT EXISTS ocr_jobs_submitted_at_idx ON ocr_jobs(submitted_at DESC)
             """)
+            # Unique constraint: prevent duplicate (author, chapter, page) submissions.
+            # Applies only when all three fields are non-null.
+            cur.execute("""
+                CREATE UNIQUE INDEX IF NOT EXISTS ocr_jobs_author_chapter_page_unique
+                ON ocr_jobs (author, chapter, page)
+                WHERE author IS NOT NULL AND chapter IS NOT NULL AND page IS NOT NULL
+            """)
         conn.commit()
         print("Database initialized.")
     except Exception as exc: