feat: add auto-fetch for missing dependencies from upstream registries

Add auto_fetch parameter to dependency resolution endpoint that fetches
missing dependencies from upstream registries (PyPI) when resolving.

- Add RegistryClient abstraction with PyPIRegistryClient implementation
- Extract fetch_and_cache_pypi_package() for reuse
- Add resolve_dependencies_with_fetch() async function
- Extend MissingDependency schema with fetch_attempted/fetch_error
- Add fetched list to DependencyResolutionResponse
- Add auto_fetch_max_depth config setting (default: 3)
- Remove Usage section from Package page UI
- Add 6 integration tests for auto-fetch functionality

backend/app/schemas.py

@@ -892,6 +892,8 @@ class MissingDependency(BaseModel):
     package: str
     constraint: Optional[str] = None
     required_by: Optional[str] = None
+    fetch_attempted: bool = False  # True if auto-fetch was attempted
+    fetch_error: Optional[str] = None  # Error message if fetch failed
 
 
 class DependencyResolutionResponse(BaseModel):
@@ -899,6 +901,7 @@ class DependencyResolutionResponse(BaseModel):
     requested: Dict[str, str]  # project, package, ref
     resolved: List[ResolvedArtifact]
     missing: List[MissingDependency] = []
+    fetched: List[ResolvedArtifact] = []  # Artifacts fetched from upstream during resolution
     total_size: int
     artifact_count: int