Add github_pr column to prs table (migration v21)

Enables GitHub↔Forgejo PR linking for the contributor pipeline.
Mirror script will store GitHub PR number when creating Forgejo PRs,
allowing back-sync of eval feedback and merge/close status.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/db.py

@@ -9,7 +9,7 @@ from . import config
 
 logger = logging.getLogger("pipeline.db")
 
-SCHEMA_VERSION = 20
+SCHEMA_VERSION = 21
 
 SCHEMA_SQL = """
 CREATE TABLE IF NOT EXISTS schema_version (
@@ -70,6 +70,7 @@ CREATE TABLE IF NOT EXISTS prs (
     last_attempt TEXT,
     cost_usd REAL DEFAULT 0,
     auto_merge INTEGER DEFAULT 0,
+    github_pr INTEGER,
     created_at TEXT DEFAULT (datetime('now')),
     merged_at TEXT
 );
@@ -546,6 +547,17 @@ def migrate(conn: sqlite3.Connection):
         conn.commit()
         logger.info("Migration v20: added conflict retry columns to prs")
 
+    if current < 21:
+        try:
+            conn.execute("ALTER TABLE prs ADD COLUMN github_pr INTEGER")
+        except sqlite3.OperationalError:
+            pass
+        conn.execute(
+            "CREATE INDEX IF NOT EXISTS idx_prs_github_pr ON prs (github_pr) WHERE github_pr IS NOT NULL"
+        )
+        conn.commit()
+        logger.info("Migration v21: added github_pr column + index to prs")
+
     if current < SCHEMA_VERSION:
         conn.execute(
             "INSERT OR REPLACE INTO schema_version (version) VALUES (?)",