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-topology/src/classifier.rs

@@ -8,12 +8,14 @@
 
 use std::collections::VecDeque;
 
+use serde::Serialize;
+
 use crate::graph::TopologyGraph;
 use crate::kind::TopologyKind;
 
 /// Objective structural metrics for a topology graph (treated undirected for
 /// connectivity, directed for hierarchy).
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq, Serialize)]
 pub struct GraphMetrics {
     /// Number of nodes.
     pub order: usize,
@@ -38,7 +40,7 @@ pub struct GraphMetrics {
 }
 
 /// The result of [`classify`].
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq, Serialize)]
 pub struct Classification {
     /// Best-matching topology kind.
     pub primary: TopologyKind,