feat(api): topology endpoints (catalog / classify / build)

Stateless, auth-gated routes backing the topology builder UI:
- GET  /api/topologies          — catalog of the 12 kinds + descriptions + role mix
- POST /api/topologies/classify — infer a kind + metrics from a posted graph
- POST /api/topologies/build    — build a canonical graph from {kind, roles}

Add Serialize to cm-topology Classification/GraphMetrics; add ApiError::BadRequest
(400) for invalid build input; add cm-topology dep. 3 integration tests
(catalog/build/auth) green against Postgres; offline build + clippy clean.

No DB or provider yet — running/comparing topologies is a later provider-backed
endpoint. Server redeploy will batch with the ReactFlow UI that consumes these.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

crates/cm-api/src/routes/mod.rs

@@ -15,3 +15,4 @@ pub mod sessions;
 pub mod skills;
 pub mod slack;
 pub mod team;
+pub mod topology;