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>

src/server/api/health.py

@@ -5,7 +5,7 @@ from datetime import datetime
 from typing import Any, Dict, Optional
 
 import psutil
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, Request
 from pydantic import BaseModel
 from sqlalchemy import text
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -28,6 +28,7 @@ class HealthStatus(BaseModel):
     anime_directory_configured: bool = False
     scheduler_next_run: Optional[str] = None
     scheduler_last_run: Optional[str] = None
+    checks: Optional[Dict[str, Any]] = None
 
 
 class DatabaseHealth(BaseModel):
@@ -173,13 +174,14 @@ def get_system_metrics() -> SystemMetrics:
 
 
 @router.get("", response_model=HealthStatus)
-async def basic_health_check() -> HealthStatus:
+async def basic_health_check(request: Request) -> HealthStatus:
     """Basic health check endpoint.
     
     This endpoint does not depend on anime_directory configuration
     and should always return 200 OK for basic health monitoring.
     Includes service information for identification.
     Includes scheduler next/last run times for monitoring tools.
+    Includes startup health check results.
 
     Returns:
         HealthStatus: Simple health status with timestamp and service info.
@@ -195,18 +197,67 @@ async def basic_health_check() -> HealthStatus:
     except Exception:
         pass
 
+    # Get startup checks from app state
+    checks = getattr(request.app.state, "startup_checks", None)
+    
+    # Determine overall status based on checks
+    overall_status = "healthy"
+    if checks:
+        for check_name, check_data in checks.items():
+            if check_data.get("status") == "error":
+                overall_status = "unhealthy"
+                break
+            elif check_data.get("status") == "warning":
+                overall_status = "degraded"
+    
     logger.debug("Basic health check requested")
     return HealthStatus(
-        status="healthy",
+        status=overall_status,
         timestamp=datetime.now().isoformat(),
         service="aniworld-api",
         series_app_initialized=_series_app is not None,
         anime_directory_configured=bool(settings.anime_directory),
         scheduler_next_run=scheduler_status.get("next_run"),
         scheduler_last_run=scheduler_status.get("last_run"),
+        checks=checks,
     )
 
 
+@router.get("/ready")
+async def ready_check(request: Request) -> Dict[str, Any]:
+    """Readiness check endpoint for container orchestrators.
+    
+    Returns 503 if critical dependencies are not available.
+    This endpoint is used by Kubernetes, Docker Swarm, etc. to determine
+    if the container should receive traffic.
+    
+    Returns:
+        dict: Readiness status with checks details.
+    """
+    checks = getattr(request.app.state, "startup_checks", {})
+    
+    critical_failures = []
+    for check_name, check_data in checks.items():
+        if check_data.get("status") == "error":
+            critical_failures.append(f"{check_name}: {check_data.get('message')}")
+    
+    if critical_failures:
+        return {
+            "status": "not_ready",
+            "ready": False,
+            "timestamp": datetime.now().isoformat(),
+            "critical_failures": critical_failures,
+            "checks": checks,
+        }
+    
+    return {
+        "status": "ready",
+        "ready": True,
+        "timestamp": datetime.now().isoformat(),
+        "checks": checks,
+    }
+
+
 @router.get("/detailed", response_model=DetailedHealthStatus)
 async def detailed_health_check(
     db: AsyncSession = Depends(get_database_session),