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.

tests/test_transport.py

@@ -0,0 +1,27 @@
+"""Transport-level tests for the mcp-maildir ASGI app.
+
+Current MCP clients (OpenCode, Claude, ...) only speak Streamable HTTP: they
+POST JSON-RPC to the configured URL. Serving the legacy SSE transport alone
+makes those clients fail with `405 Method Not Allowed`, so the app built by
+`server.build_app()` must expose the MCP endpoint at /mcp.
+
+Env vars are set in tests/conftest.py (shared session-wide).
+"""
+
+from server import build_app, MCP_HTTP_PATH
+
+
+def app_paths() -> set:
+    return {getattr(route, "path", None) for route in build_app().routes}
+
+
+class TestTransport:
+    def test_mcp_endpoint_is_streamable_http(self):
+        assert MCP_HTTP_PATH == "/mcp"
+        assert MCP_HTTP_PATH in app_paths()
+
+    def test_health_endpoint_is_exposed_for_probes(self):
+        assert "/health" in app_paths()
+
+    def test_legacy_sse_endpoint_is_gone(self):
+        assert "/sse" not in app_paths()