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>

tests/unit/test_health.py

@@ -1,6 +1,6 @@
 """Unit tests for health check endpoints."""
 
-from unittest.mock import AsyncMock, patch
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
@@ -12,16 +12,20 @@ from src.server.api.health import (
     check_database_health,
     check_filesystem_health,
     get_system_metrics,
+    ready_check,
 )
 
 
 @pytest.mark.asyncio
-async def test_basic_health_check():
-    """Test basic health check endpoint."""
+async def test_basic_health_check_no_startup_checks():
+    """Test basic health check endpoint with no startup checks."""
+    mock_request = MagicMock()
+    mock_request.app.state.startup_checks = {}
+    
     with patch("src.config.settings.settings") as mock_settings, \
          patch("src.server.utils.dependencies._series_app", None):
         mock_settings.anime_directory = ""
-        result = await basic_health_check()
+        result = await basic_health_check(mock_request)
 
         assert isinstance(result, HealthStatus)
         assert result.status == "healthy"
@@ -32,6 +36,85 @@ async def test_basic_health_check():
         assert result.anime_directory_configured is False
 
 
+@pytest.mark.asyncio
+async def test_basic_health_check_with_error_check():
+    """Test basic health check reflects error status from startup checks."""
+    mock_request = MagicMock()
+    mock_request.app.state.startup_checks = {
+        "anime_directory": {"status": "error", "message": "not configured", "path": None},
+        "ffmpeg": {"status": "ok", "message": "Found at /usr/bin/ffmpeg"},
+        "dns_aniworld": {"status": "ok", "message": "Resolved successfully"},
+        "dns_tmdb": {"status": "ok", "message": "Resolved successfully"},
+    }
+    
+    with patch("src.config.settings.settings") as mock_settings, \
+         patch("src.server.utils.dependencies._series_app", None):
+        mock_settings.anime_directory = ""
+        result = await basic_health_check(mock_request)
+
+        assert isinstance(result, HealthStatus)
+        assert result.status == "unhealthy"
+        assert result.checks is not None
+        assert result.checks["anime_directory"]["status"] == "error"
+
+
+@pytest.mark.asyncio
+async def test_basic_health_check_with_warning_only():
+    """Test basic health check shows degraded when only warnings present."""
+    mock_request = MagicMock()
+    mock_request.app.state.startup_checks = {
+        "anime_directory": {"status": "ok", "message": "Found", "path": "/anime"},
+        "ffmpeg": {"status": "warning", "message": "not found in PATH"},
+        "dns_aniworld": {"status": "ok", "message": "Resolved successfully"},
+        "dns_tmdb": {"status": "ok", "message": "Resolved successfully"},
+    }
+    
+    with patch("src.config.settings.settings") as mock_settings, \
+         patch("src.server.utils.dependencies._series_app", None):
+        mock_settings.anime_directory = "/anime"
+        result = await basic_health_check(mock_request)
+
+        assert isinstance(result, HealthStatus)
+        assert result.status == "degraded"
+
+
+@pytest.mark.asyncio
+async def test_ready_check_all_healthy():
+    """Test ready check returns ready when all checks pass."""
+    mock_request = MagicMock()
+    mock_request.app.state.startup_checks = {
+        "anime_directory": {"status": "ok", "message": "Found", "path": "/anime"},
+        "ffmpeg": {"status": "ok", "message": "Found at /usr/bin/ffmpeg"},
+        "dns_aniworld": {"status": "ok", "message": "Resolved successfully"},
+        "dns_tmdb": {"status": "ok", "message": "Resolved successfully"},
+    }
+    
+    result = await ready_check(mock_request)
+    
+    assert result["ready"] is True
+    assert result["status"] == "ready"
+    assert "critical_failures" not in result
+
+
+@pytest.mark.asyncio
+async def test_ready_check_with_critical_failure():
+    """Test ready check returns not_ready when anime_directory not configured."""
+    mock_request = MagicMock()
+    mock_request.app.state.startup_checks = {
+        "anime_directory": {"status": "error", "message": "not configured", "path": None},
+        "ffmpeg": {"status": "warning", "message": "not found in PATH"},
+        "dns_aniworld": {"status": "ok", "message": "Resolved successfully"},
+        "dns_tmdb": {"status": "ok", "message": "Resolved successfully"},
+    }
+    
+    result = await ready_check(mock_request)
+    
+    assert result["ready"] is False
+    assert result["status"] == "not_ready"
+    assert len(result["critical_failures"]) == 1
+    assert "anime_directory" in result["critical_failures"][0]
+
+
 @pytest.mark.asyncio
 async def test_database_health_check_success():
     """Test database health check with successful connection."""