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_startup_health_checks.py

@@ -0,0 +1,135 @@
+"""Unit tests for startup health checks in fastapi_app.py."""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+class TestStartupHealthChecks:
+    """Test startup health check function."""
+
+    @pytest.mark.asyncio
+    async def test_ffmpeg_missing_sets_warning(self):
+        """Test ffmpeg missing results in warning status."""
+        mock_logger = MagicMock()
+        
+        with patch("shutil.which", return_value=None):
+            from src.server.fastapi_app import _run_startup_health_checks
+            result = await _run_startup_health_checks(mock_logger)
+            
+            assert result["ffmpeg"]["status"] == "warning"
+            assert "not found in PATH" in result["ffmpeg"]["message"]
+
+    @pytest.mark.asyncio
+    async def test_ffmpeg_present_sets_ok(self):
+        """Test ffmpeg present results in ok status."""
+        mock_logger = MagicMock()
+        
+        with patch("shutil.which", return_value="/usr/bin/ffmpeg"):
+            from src.server.fastapi_app import _run_startup_health_checks
+            result = await _run_startup_health_checks(mock_logger)
+            
+            assert result["ffmpeg"]["status"] == "ok"
+            assert "Found at" in result["ffmpeg"]["message"]
+
+    @pytest.mark.asyncio
+    async def test_anime_directory_not_configured_sets_error(self):
+        """Test anime_directory not configured results in error status."""
+        mock_logger = MagicMock()
+        
+        with patch("src.config.settings.settings") as mock_settings:
+            mock_settings.anime_directory = ""
+            
+            from src.server.fastapi_app import _run_startup_health_checks
+            result = await _run_startup_health_checks(mock_logger)
+            
+            assert result["anime_directory"]["status"] == "error"
+            assert result["anime_directory"]["path"] is None
+            assert "not configured" in result["anime_directory"]["message"]
+
+    @pytest.mark.asyncio
+    async def test_anime_directory_not_exists_sets_error(self):
+        """Test anime_directory path not existing results in error status."""
+        mock_logger = MagicMock()
+        
+        with patch("src.config.settings.settings") as mock_settings:
+            mock_settings.anime_directory = "/nonexistent/path"
+            
+            with patch("os.path.isdir", return_value=False):
+                from src.server.fastapi_app import _run_startup_health_checks
+                result = await _run_startup_health_checks(mock_logger)
+                
+                assert result["anime_directory"]["status"] == "error"
+                assert "does not exist" in result["anime_directory"]["message"]
+
+    @pytest.mark.asyncio
+    async def test_anime_directory_not_writable_sets_error(self):
+        """Test anime_directory not writable results in error status."""
+        mock_logger = MagicMock()
+        
+        with patch("src.config.settings.settings") as mock_settings:
+            mock_settings.anime_directory = "/some/path"
+            
+            with patch("os.path.isdir", return_value=True):
+                with patch("os.access", return_value=False):
+                    from src.server.fastapi_app import _run_startup_health_checks
+                    result = await _run_startup_health_checks(mock_logger)
+                    
+                    assert result["anime_directory"]["status"] == "error"
+                    assert "not writable" in result["anime_directory"]["message"]
+
+    @pytest.mark.asyncio
+    async def test_anime_directory_ok_when_writable(self):
+        """Test anime_directory exists and writable results in ok status."""
+        mock_logger = MagicMock()
+        
+        with patch("src.config.settings.settings") as mock_settings:
+            mock_settings.anime_directory = "/valid/path"
+            
+            with patch("os.path.isdir", return_value=True):
+                with patch("os.access", return_value=True):
+                    from src.server.fastapi_app import _run_startup_health_checks
+                    result = await _run_startup_health_checks(mock_logger)
+                    
+                    assert result["anime_directory"]["status"] == "ok"
+
+    @pytest.mark.asyncio
+    async def test_dns_aniworld_failure_sets_warning(self):
+        """Test DNS failure for aniworld.to sets warning status."""
+        mock_logger = MagicMock()
+        
+        import socket
+        with patch("socket.gethostbyname", side_effect=socket.gaierror("DNS failed")):
+            from src.server.fastapi_app import _run_startup_health_checks
+            result = await _run_startup_health_checks(mock_logger)
+            
+            assert result["dns_aniworld"]["status"] == "warning"
+            assert "DNS resolution failed" in result["dns_aniworld"]["message"]
+
+    @pytest.mark.asyncio
+    async def test_dns_tmdb_failure_sets_warning(self):
+        """Test DNS failure for api.themoviedb.org sets warning status."""
+        mock_logger = MagicMock()
+        
+        import socket
+        with patch("socket.gethostbyname", side_effect=socket.gaierror("DNS failed")):
+            from src.server.fastapi_app import _run_startup_health_checks
+            result = await _run_startup_health_checks(mock_logger)
+            
+            assert result["dns_tmdb"]["status"] == "warning"
+
+    @pytest.mark.asyncio
+    async def test_all_checks_returned(self):
+        """Test all health checks are present in result."""
+        mock_logger = MagicMock()
+        
+        with patch("src.config.settings.settings") as mock_settings:
+            mock_settings.anime_directory = ""
+            
+            from src.server.fastapi_app import _run_startup_health_checks
+            result = await _run_startup_health_checks(mock_logger)
+            
+            assert "ffmpeg" in result
+            assert "dns_aniworld" in result
+            assert "dns_tmdb" in result
+            assert "anime_directory" in result