Add robust PyPI dependency caching with task queue

Replace unbounded thread spawning with managed worker pool:
- New pypi_cache_tasks table tracks caching jobs
- Thread pool with 5 workers (configurable via ORCHARD_PYPI_CACHE_WORKERS)
- Automatic retries with exponential backoff (30s, 60s, then fail)
- Deduplication to prevent duplicate caching attempts

New API endpoints for visibility and control:
- GET /pypi/cache/status - queue health summary
- GET /pypi/cache/failed - list failed tasks with errors
- POST /pypi/cache/retry/{package} - retry single package
- POST /pypi/cache/retry-all - retry all failed packages

This fixes silent failures in background dependency caching where
packages would fail to cache without any tracking or retry mechanism.

backend/app/config.py

@@ -64,6 +64,11 @@ class Settings(BaseSettings):
     # Global cache settings override (None = use DB value, True/False = override DB)
     cache_auto_create_system_projects: Optional[bool] = None  # Override auto_create_system_projects
 
+    # PyPI Cache Worker settings
+    pypi_cache_workers: int = 5  # Number of concurrent cache workers
+    pypi_cache_max_depth: int = 10  # Maximum recursion depth for dependency caching
+    pypi_cache_max_attempts: int = 3  # Maximum retry attempts for failed cache tasks
+
     # JWT Authentication settings (optional, for external identity providers)
     jwt_enabled: bool = False  # Enable JWT token validation
     jwt_secret: str = ""  # Secret key for HS256, or leave empty for RS256 with JWKS
@@ -88,6 +93,24 @@ class Settings(BaseSettings):
     def is_production(self) -> bool:
         return self.env.lower() == "production"
 
+    @property
+    def PORT(self) -> int:
+        """Alias for server_port for compatibility."""
+        return self.server_port
+
+    # Uppercase aliases for PyPI cache settings (for backward compatibility)
+    @property
+    def PYPI_CACHE_WORKERS(self) -> int:
+        return self.pypi_cache_workers
+
+    @property
+    def PYPI_CACHE_MAX_DEPTH(self) -> int:
+        return self.pypi_cache_max_depth
+
+    @property
+    def PYPI_CACHE_MAX_ATTEMPTS(self) -> int:
+        return self.pypi_cache_max_attempts
+
     class Config:
         env_prefix = "ORCHARD_"
         case_sensitive = False