fix(db): add missing legacy_key_cleanup_completed column migration

Add migrate_schema_if_needed() to handle adding missing columns to existing
tables for backward compatibility. Automatically adds legacy_key_cleanup_completed
BOOLEAN column to system_settings table if missing, preventing 'no such column'
errors on startup for existing databases.

src/server/database/init.py

@@ -119,6 +119,11 @@ async def initialize_database(
             result["tables_created"] = tables
             logger.info("Created %s tables", len(tables))
         
+        # Migrate schema if needed (add missing columns to existing tables)
+        migrations = await migrate_schema_if_needed(engine)
+        if migrations:
+            logger.info("Applied %s schema migrations", len(migrations))
+        
         # Validate schema if requested
         if validate_schema:
             validation = await validate_database_schema(engine)
@@ -305,6 +310,66 @@ async def validate_database_schema(
         }
 
 
+# =============================================================================
+# Schema Migration
+# =============================================================================
+
+
+async def migrate_schema_if_needed(
+    engine: Optional[AsyncEngine] = None
+) -> List[str]:
+    """Migrate database schema to current version if needed.
+    
+    Handles adding missing columns to existing tables for backward
+    compatibility with older database schemas.
+    
+    Args:
+        engine: Optional database engine (uses default if not provided)
+    
+    Returns:
+        List of migration operations performed
+    """
+    if engine is None:
+        engine = get_engine()
+    
+    migrations_applied = []
+    
+    try:
+        async with engine.connect() as conn:
+            # Get existing columns in system_settings table
+            existing_columns = await conn.run_sync(
+                lambda sync_conn: [
+                    col["name"]
+                    for col in inspect(sync_conn).get_columns("system_settings")
+                ]
+            )
+            
+            # Migration: Add legacy_key_cleanup_completed column if missing
+            if "legacy_key_cleanup_completed" not in existing_columns:
+                logger.info(
+                    "Migrating system_settings table: "
+                    "adding legacy_key_cleanup_completed column"
+                )
+                await conn.execute(
+                    text("""
+                        ALTER TABLE system_settings
+                        ADD COLUMN legacy_key_cleanup_completed BOOLEAN
+                        NOT NULL DEFAULT 0
+                    """)
+                )
+                migrations_applied.append("added legacy_key_cleanup_completed")
+                logger.info(
+                    "Migration complete: added legacy_key_cleanup_completed column"
+                )
+    
+    except Exception as e:
+        logger.error("Schema migration failed: %s", e)
+        # Don't raise - migration failures shouldn't block startup
+        # The missing column will be handled gracefully by the application
+    
+    return migrations_applied
+
+
 # =============================================================================
 # Schema Version Management
 # =============================================================================