feat: Phase 4 — RelationshipService, postgres RelRepo, converged edges

Problem: relationship create/end existed as three drifted copies
(HTTP CreateRelationship/EndRelationship, MCP create_relationship/
end_relationship) with inline SQL, no ontology edge validation on
either path, and no audit on the MCP path.

Change:
- Internal/adapters/postgres/repositories.go: RelRepo implements
  ports.RelationshipRepository (Create/End/ListFor) over the pool,
  with in-tx upsert + audit/event side effects on Create.
- Internal/core/app/relationships.go: RelationshipService validates
  edges against the cached ontology TypeTree (tree.ValidateEdge) and
  delegates the tx to the repository. The adapter resolves slug→entity
  and extracts types before calling the service.
- HTTP CreateRelationship: resolves source/target via ReadModels,
  passes resolved types to RelationshipService for edge validation.
  EndRelationship calls the service directly (audit stays in the
  adapter for End — a simple toggle with no ontology check).
- MCP create_relationship/end_relationship: rewired to the service
  (pool resolves entity IDs inline for the tool handlers; the service
  validates edges and writes audit). The MCP path now gets ontology
  validation and audit coverage for the first time.
- Composition root: RelationshipService built with RelRepo + Ontology
  and wired through httpapi.NewHandler, ListenAndServe, and MCP
  constructors.

Verification: go build/vet, full test suite (19 pkgs, DB integration
postgres+mcp green).

internal/httpapi/api_test.go

@@ -97,7 +97,7 @@ func newTestHandler(t *testing.T, cfg config.Config) http.Handler {
 
 	repo := db.NewEntityRepo(pool)
 	onto := db.NewOntologyRepo(pool, time.Minute)
-	return NewHandler(handlerCtx, pool, cfg, app.NewEntityService(repo, onto), repo, db.NewEntityReader(pool))
+	return NewHandler(handlerCtx, pool, cfg, app.NewEntityService(repo, onto), repo, db.NewEntityReader(pool), app.NewRelationshipService(db.NewRelRepo(pool), onto))
 }
 
 // testAuthToken is the static bearer token devConfig() configures. There is