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/alembic/versions/0007_instance_container_name.py

@@ -0,0 +1,28 @@
+"""add container_name to tool_instances
+
+Revision ID: 0007_instance_container_name
+Revises: 0006_tool_instances
+Create Date: 2026-05-20 08:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision: str = "0007_instance_container_name"
+down_revision: Union[str, None] = "0006_tool_instances"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "tool_instances",
+        sa.Column("container_name", sa.String(255), nullable=True)
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("tool_instances", "container_name")