feat: add external MCP server support with streamable-http proxy

Support non-containerized MCP servers via externalUrl field and add
streamable-http session management for HA MCP proof of concept.

- Add externalUrl, command, containerPort fields to McpServer schema
- Skip Docker orchestration for external servers (virtual instances)
- Implement streamable-http proxy with Mcp-Session-Id session management
- Parse SSE-framed responses from streamable-http endpoints
- Add command passthrough to Docker container creation
- Create HA MCP example manifest (examples/ha-mcp.yaml)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/db/prisma/migrations/migration_lock.toml

@@ -0,0 +1,3 @@
+# Please do not edit this file manually
+# It should be added in your version-control system (e.g., Git)
+provider = "postgresql"