Add versioned DB migration system with automatic backup

migrations.py
- schema_migrations table tracks applied versions (version, description, applied_at)
- MIGRATIONS list is append-only; each entry is (version, description, sql)
- backup() dumps all user-data tables to a timestamped JSON file in backups/
  before any schema changes so data can be recovered if something goes wrong
- run_migrations() is idempotent: already-applied versions are skipped

Integration
- app.py calls _run_startup_migrations() at module load so every restart
  applies any pending migrations (no-op if schema is current)
- setup_routes.py calls run_migrations() after the initial setup form is
  submitted so all tables exist before the user hits the main page for the
  first time
- notes.py and admin.py: removed all per-request CREATE TABLE DDL; schema is
  now owned entirely by the migration system

Docker
- docker-compose.dev.yml: add backups-data volume so JSON backups survive
  container restarts and rebuilds
- Dockerfile: pre-create /app/backend/logs and /app/backend/backups so the
  directories exist even before volumes are mounted

Adding future schema changes
- Append a new (version, description, sql) tuple to MIGRATIONS in migrations.py
- The next restart will detect it as pending, back up first, then apply it

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

backend/util/setup_routes.py

@@ -43,6 +43,18 @@ def setup():
         reset_pool()
         if test_connection(db_config):
             initialize_admin_user(db_config)
+            # Apply schema migrations immediately so all tables exist before
+            # the user lands on the main page.
+            try:
+                from util.db_pool import get_connection
+                from util.migrations import run_migrations
+                conn = get_connection()
+                try:
+                    run_migrations(conn)
+                finally:
+                    conn.close()
+            except Exception as e:
+                logger.warning(f"[setup] Post-setup migration error: {e}")
             return redirect('/')
         else:
             return "Verbindung fehlgeschlagen. Bitte zurück und prüfen.", 500