feat(instance-proxy): add HTTP proxy for tool instances

Add API proxy endpoint so users can access running tool instances
through the backend API instead of internal Docker network.

Backend:
- Add container_name field to ToolInstance model
- Create /instances/{id}/proxy/{path:path} endpoint with ownership checks
- Proxy HTTP requests to containers via docker network using container names
- Support all HTTP methods (GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS)
- Store proxy URL in instance.url instead of localhost
- Add Alembic migration 0007 for container_name column
- Add get_container_name() utility to docker.py

Frontend:
- Update Open button to use full proxy URL (API_BASE_URL + instance.url)

Closes instance-proxy OpenSpec change.

apps/api/src/models/tool_instance.py

@@ -38,6 +38,9 @@ class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     container_id: Mapped[str | None] = mapped_column(
         String(255), nullable=True
     )
+    container_name: Mapped[str | None] = mapped_column(
+        String(255), nullable=True
+    )
     compose_path: Mapped[str | None] = mapped_column(
         String(1024), nullable=True
     )