Add startup health checks and /health/ready endpoint

- Add _run_startup_health_checks() function in fastapi_app.py
  - Check ffmpeg availability (warning)
  - Check DNS resolution for aniworld.to and api.themoviedb.org (warning)
  - Check anime_directory configuration and writability (error)
- Store startup checks in app.state for health endpoint access
- Add /health/ready endpoint for container orchestrators
  - Returns not_ready with 503 when critical failures present
  - Includes critical_failures list for debugging
- Update /health endpoint to include startup check results
  - Status reflects worst check (error > warning > ok)
- Document health check endpoints in DEVELOPMENT.md
- Add unit tests for startup health checks
- Add unit tests for /health/ready endpoint

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/DEVELOPMENT.md

@@ -165,6 +165,61 @@ scheduler = AsyncIOScheduler(jobstores=jobstores)
 
 **If server is down >1 hour:** No automatic recovery. Manual trigger via `POST /api/scheduler/trigger-rescan` or wait for next scheduled run.
 
+### Health Check Endpoints
+
+The application provides health check endpoints for monitoring and container orchestration:
+
+#### `GET /health`
+Basic health check returning service status and startup health check results.
+
+**Response fields:**
+- `status`: "healthy", "degraded", or "unhealthy" based on startup checks
+- `timestamp`: ISO timestamp of the check
+- `series_app_initialized`: Whether the series app is loaded
+- `anime_directory_configured`: Whether anime_directory is set
+- `scheduler_next_run` / `scheduler_last_run`: Scheduler times
+- `checks`: Detailed startup check results (ffmpeg, DNS, anime_directory)
+
+#### `GET /health/ready`
+Readiness check for container orchestrators (Kubernetes, Docker Swarm).
+
+**Response when ready:**
+```json
+{
+  "status": "ready",
+  "ready": true,
+  "timestamp": "2024-01-01T00:00:00",
+  "checks": {...}
+}
+```
+
+**Response when not ready (503):**
+```json
+{
+  "status": "not_ready",
+  "ready": false,
+  "timestamp": "2024-01-01T00:00:00",
+  "critical_failures": ["anime_directory: not configured"],
+  "checks": {...}
+}
+```
+
+#### `GET /health/detailed`
+Comprehensive health check including database, filesystem, and system metrics.
+
+#### Startup Health Checks
+
+On application startup, the following checks are performed:
+
+| Check | Failure Status | Impact |
+|-------|---------------|--------|
+| `ffmpeg` | warning | HLS downloads may fail |
+| `dns_aniworld` | warning | Provider requests may fail |
+| `dns_tmdb` | warning | TMDB API calls may fail |
+| `anime_directory` | error | Download service disabled |
+
+DNS checks are warnings because failures can be transient. anime_directory errors disable the download service to prevent failures.
+
 ### Troubleshooting Development Issues
 
 #### Scheduler missed a run
@@ -175,3 +230,21 @@ scheduler = AsyncIOScheduler(jobstores=jobstores)
 4. Trigger manually: `POST /api/scheduler/trigger-rescan`
 5. Monitor next run: `GET /health` → `scheduler_next_run`
 6. If problem repeats, increase `misfire_grace_time` in `scheduler_service.py`.
+
+#### Startup health check failures
+
+If `/health` returns `unhealthy` status:
+
+1. **anime_directory error**: Directory not configured or not writable
+   - Check `ANIME_DIRECTORY` environment variable
+   - Verify directory exists and permissions allow write access
+   - Download service will not initialize until resolved
+
+2. **ffmpeg warning**: ffmpeg not found in PATH
+   - HLS stream downloads will fail
+   - Install ffmpeg: `apt install ffmpeg` or `brew install ffmpeg`
+
+3. **DNS warnings**: Domain resolution failed
+   - Check network connectivity
+   - DNS failures are transient — warnings don't block startup
+   - Retry later to verify: `GET /health`