Add GET /files/{filename} route (behind auth) that proxies files from
Gitea raw URLs with in-memory caching. Uses SHA-based cache invalidation:
checks Gitea commit API every 5 min, only re-downloads when file changes.
- internal/handlers/files.go: proxy handler with SHA-based cache
- POST /api/files/refresh: cache-bust endpoint
- GITEA_TOKEN env var for private repo access
- Download card on landing page with i18n DE/EN
77061b2708