docs: update deployment docs for CI timeouts, build info, and prod safety

- hetzner-server-setup: runner timeout 3h, shutdown_timeout 300s,
  deploy.sh now writes .build-info and uses explicit -f flag
- gitea: document unit-only CI tests and xdist incompatibility
- docker: add build info section, document volume mount approach

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

docs/deployment/docker.md

@@ -331,6 +331,25 @@ docker compose -f docker-compose.prod.yml exec api python scripts/seed/init_prod
 
 ---
 
+## Build Info
+
+The deploy script writes a `.build-info` JSON file (commit SHA + deploy timestamp) before rebuilding containers. This file is mounted as a read-only volume into the API container:
+
+```yaml
+# In docker-compose.yml
+volumes:
+  - ./.build-info:/app/.build-info:ro
+```
+
+The app reads it via `app/core/build_info.py` and exposes it in:
+
+- **`/health` endpoint** — `commit` and `deployed_at` fields
+- **Admin sidebar** — version, commit, and deploy timestamp
+
+In local development (where `.build-info` doesn't exist), the app falls back to `git rev-parse` for the commit SHA.
+
+---
+
 ## Daily Operations
 
 ### View Logs