fix: serve MCP over Streamable HTTP instead of legacy SSE

Current MCP clients (OpenCode, Claude, ...) only speak Streamable HTTP: they
POST JSON-RPC to the configured URL, so a legacy SSE endpoint answers
405 Method Not Allowed and not a single tool can be reached.

Run FastMCP's http_app (build_app()) under uvicorn instead of mcp.run(sse):
the MCP endpoint is now /mcp, and the custom GET /health route keeps serving
the Kubernetes probes.

Add transport tests asserting /mcp and /health are exposed and /sse is gone.

docker-compose.yml

@@ -15,7 +15,7 @@ services:
       dockerfile: pkg/Dockerfile
     container_name: mcp_maildir_server
     ports:
-      - "127.0.0.1:8000:8000" # Expose the MCP HTTP (SSE) server
+      - "127.0.0.1:8000:8000" # Expose the MCP Streamable HTTP server (/mcp)
     env_file:
       - .env
     environment: