Backend Sprint 2+3: Health-Modul, Multi-Dog Tagebuch, Pillow, Migrations

- database.py: diary_dogs + walk_participant_dogs Tabellen, idempotente
  Migration für Health-Felder (charge_nr, kosten, diagnose, …), Backfill
- routes/health.py: vollständiges Health-Modul (war Stub), CRUD für
  Impfung/Entwurmung/Tierarzt/Medikament/Gewicht/Allergie/Dokument
- routes/diary.py: Multi-Dog n:m via diary_dogs (dog_ids in allen Endpoints)
- routes/dogs.py: Foto-Upload konvertiert HEIC/PNG/WebP → JPEG via Pillow
- routes/poison.py: Resolve mit Grundauswahl + Soft-Delete (geloest_von/at/grund)
- ki.py: health_summary() für KI-Gesundheitsbericht
- main.py: /favicon.ico Route
- requirements.txt: Pillow 11.2.1 + pillow-heif 0.22.0

backend/main.py

@@ -89,6 +89,10 @@ MEDIA_DIR = os.getenv("MEDIA_DIR", "/data/media")
 os.makedirs(MEDIA_DIR, exist_ok=True)
 app.mount("/media", StaticFiles(directory=MEDIA_DIR), name="media")
 
+@app.get("/favicon.ico")
+async def favicon():
+    return FileResponse(f"{STATIC_DIR}/icons/favicon.ico")
+
 @app.get("/manifest.json")
 async def manifest():
     return FileResponse(f"{STATIC_DIR}/manifest.json")