Add cache/resolve endpoint and reduce footer padding

- Add POST /api/v1/cache/resolve endpoint that caches artifacts by package
  coordinates (source_type, package, version) instead of requiring a URL
- Server finds download URL from configured upstream sources (PyPI supported)
- Reduces footer padding from 24px to 12px for cleaner layout

backend/app/schemas.py

@@ -1432,4 +1432,41 @@ class CacheResponse(BaseModel):
     user_reference: Optional[str] = None  # e.g., "my-app/npm-deps:lodash-4.17.21"
 
 
+class CacheResolveRequest(BaseModel):
+    """Request to cache an artifact by package coordinates (no URL required).
+
+    The server will construct the appropriate URL based on source_type and
+    configured upstream sources.
+    """
+    source_type: str
+    package: str
+    version: str
+    user_project: Optional[str] = None
+    user_package: Optional[str] = None
+    user_tag: Optional[str] = None
+
+    @field_validator('source_type')
+    @classmethod
+    def validate_source_type(cls, v: str) -> str:
+        if v not in SOURCE_TYPES:
+            raise ValueError(f"source_type must be one of: {', '.join(SOURCE_TYPES)}")
+        return v
+
+    @field_validator('package')
+    @classmethod
+    def validate_package(cls, v: str) -> str:
+        v = v.strip()
+        if not v:
+            raise ValueError("package cannot be empty")
+        return v
+
+    @field_validator('version')
+    @classmethod
+    def validate_version(cls, v: str) -> str:
+        v = v.strip()
+        if not v:
+            raise ValueError("version cannot be empty")
+        return v
+
+