fix: Auto-Migrate fuer fehlende DB-Spalten beim App-Start

Behebt 500 Internal Server Error beim Erstellen von Share-Links
auf bestehenden Datenbanken, denen die neue 'permission'-Spalte fehlt.

Problem: db.create_all() erstellt nur neue Tabellen, fuegt aber keine
Spalten zu bestehenden Tabellen hinzu. Wenn das Model neue Spalten
bekommt, crasht die App auf alten Datenbanken.

Loesung: _auto_migrate() vergleicht beim Start jede Model-Definition
mit dem tatsaechlichen DB-Schema (via SQLAlchemy Inspector) und fuegt
fehlende Spalten per ALTER TABLE ADD COLUMN hinzu. Funktioniert fuer
alle Datentypen mit korrekten Defaults (VARCHAR, INTEGER, BOOLEAN, etc.).

Output im Log: [Auto-Migrate] Added column share_links.permission (VARCHAR(20))

Das macht die App update-sicher - neue Spalten in Models werden
automatisch zur bestehenden DB hinzugefuegt, ohne manuelles Migrieren.

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

backend/app/__init__.py

@@ -8,6 +8,58 @@ from app.config import Config
 from app.extensions import db, bcrypt, migrate
 
 
+def _auto_migrate(db):
+    """Add missing columns to existing tables by comparing model definitions
+    with actual database schema. This handles the case where new columns are
+    added to models but db.create_all() only creates new tables."""
+    import sqlalchemy
+
+    with db.engine.connect() as conn:
+        inspector = sqlalchemy.inspect(db.engine)
+
+        for table_name, table in db.metadata.tables.items():
+            if not inspector.has_table(table_name):
+                continue
+
+            existing_cols = {col['name'] for col in inspector.get_columns(table_name)}
+
+            for column in table.columns:
+                if column.name not in existing_cols:
+                    # Determine SQL type
+                    col_type = column.type.compile(db.engine.dialect)
+
+                    # Determine default value
+                    default = ''
+                    if column.default is not None:
+                        if hasattr(column.default, 'arg'):
+                            val = column.default.arg
+                            if callable(val):
+                                default = ''
+                            elif isinstance(val, str):
+                                default = f" DEFAULT '{val}'"
+                            elif isinstance(val, bool):
+                                default = f" DEFAULT {1 if val else 0}"
+                            elif isinstance(val, (int, float)):
+                                default = f" DEFAULT {val}"
+                    elif column.nullable:
+                        default = ' DEFAULT NULL'
+                    elif col_type.upper().startswith(('VARCHAR', 'TEXT')):
+                        default = " DEFAULT ''"
+                    elif col_type.upper().startswith(('INTEGER', 'BIGINT', 'FLOAT')):
+                        default = " DEFAULT 0"
+                    elif col_type.upper() == 'BOOLEAN':
+                        default = " DEFAULT 0"
+
+                    sql = f'ALTER TABLE "{table_name}" ADD COLUMN "{column.name}" {col_type}{default}'
+                    try:
+                        conn.execute(db.text(sql))
+                        print(f'[Auto-Migrate] Added column {table_name}.{column.name} ({col_type})')
+                    except Exception as e:
+                        print(f'[Auto-Migrate] Failed to add {table_name}.{column.name}: {e}')
+
+        conn.commit()
+
+
 def create_app(config_class=Config):
     # Check if static frontend build exists (Docker production mode)
     static_dir = Path(__file__).resolve().parent.parent / 'static'
@@ -61,11 +113,14 @@ def create_app(config_class=Config):
         def serve_spa(e):
             return send_from_directory(str(static_dir), 'index.html')
 
-    # Create tables
+    # Create tables + auto-migrate missing columns
     with app.app_context():
         from app import models  # noqa: F401
         db.create_all()
 
+        # Auto-migrate: add missing columns to existing tables
+        _auto_migrate(db)
+
         # Enable WAL mode for SQLite
         with db.engine.connect() as conn:
             conn.execute(db.text('PRAGMA journal_mode=WAL'))