chore: beta release v0.5.0-beta

- Bump version to 0.5.0-beta
- Add Apache-2.0 LICENSE (official text) + NOTICE
- Add Docker support: multi-stage Dockerfile + docker-compose
  * non-root user, tini PID 1, gunicorn + uvicorn workers
  * healthcheck, OCI labels, persistent /data volume
- Add GitHub Actions CI (lint/import-check + Docker smoke-test)
- Backend polish:
  * modern lifespan handler (replaces deprecated on_event)
  * explicit sort validation (400 instead of 500)
  * title min_length=1 schema validation
  * configurable data dir + http timeout via env
  * avg_rating helper extracted (DRY)
- Frontend polish:
  * beta badge in title, footer with live version
  * loading spinner on initial fetch
  * updated README, badges, config table, roadmap

app/seed.py

@@ -1,4 +1,5 @@
 """Legt ein paar Demo-Datensätze an, falls die DB leer ist."""
+# SPDX-License-Identifier: Apache-2.0
 from __future__ import annotations
 
 import logging
@@ -6,7 +7,7 @@ from datetime import date
 
 from sqlalchemy.orm import Session
 
-from app import crud, schemas
+from app import crud, models, schemas
 
 log = logging.getLogger(__name__)
 
@@ -120,7 +121,7 @@ DEMO_SERIES = [
 
 
 def seed_if_empty(db: Session) -> None:
-    if db.query(__import__("app").models.Media).count() > 0:  # type: ignore[attr-defined]
+    if db.query(models.Media).count() > 0:
         return
     log.info("Leere DB – lege Demo-Daten an.")
     for spec in DEMO_BOOKS: