fix: only process numbered migrations (XXX_*.sql pattern)

Modified run_migrations() to only process files matching pattern: \d{3}_*.sql
This prevents utility scripts (check_features.sql) and manually applied
migrations (v9c_*.sql) from being executed.

Only properly numbered migrations like 003_add_email_verification.sql
will be processed.

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

backend/db_init.py

@@ -155,6 +155,7 @@ def apply_migration(filepath, filename):
 def run_migrations(migrations_dir="/app/migrations"):
     """Run all pending migrations."""
     import glob
+    import re
 
     if not os.path.exists(migrations_dir):
         print("✓ No migrations directory found")
@@ -167,8 +168,10 @@ def run_migrations(migrations_dir="/app/migrations"):
     # Get already applied migrations
     applied = get_applied_migrations()
 
-    # Get all migration files
-    migration_files = sorted(glob.glob(os.path.join(migrations_dir, "*.sql")))
+    # Get all migration files (only numbered migrations like 001_*.sql)
+    all_files = sorted(glob.glob(os.path.join(migrations_dir, "*.sql")))
+    migration_pattern = re.compile(r'^\d{3}_.*\.sql$')
+    migration_files = [f for f in all_files if migration_pattern.match(os.path.basename(f))]
 
     if not migration_files:
         print("✓ No migration files found")