fix: migration runner strips BEGIN/COMMIT and guards missing tables

Root cause: migrations 046-047 used explicit BEGIN/COMMIT which
conflicts with psycopg2 implicit transactions, and ALTER TABLE
on canonical_controls fails when the table doesn't exist on
production. This blocked all subsequent migrations (048-053).

Changes:
- migration_runner.py: strip BEGIN/COMMIT from SQL before executing
- 046: wrap canonical_controls ALTER in DO $$ IF EXISTS block
- 047: wrap canonical_controls ALTER in DO $$ IF EXISTS block

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

backend-compliance/migration_runner.py

@@ -84,7 +84,9 @@ def run_migrations():
             logger.info("Applying migration: %s", migration_file.name)
             try:
                 sql = migration_file.read_text(encoding="utf-8")
-                # Execute the full SQL file as-is (supports BEGIN/COMMIT)
+                # Strip explicit BEGIN/COMMIT — we manage transactions ourselves
+                sql = re.sub(r'(?mi)^\s*BEGIN\s*;\s*$', '', sql)
+                sql = re.sub(r'(?mi)^\s*COMMIT\s*;\s*$', '', sql)
                 cursor.execute(sql)
                 raw_conn.commit()
                 # Record successful application