feat(graph): add Postgres graph runtime repository + compatibility endpoints

Introduces IGraphRuntimeRepository + PostgresGraphRuntimeRepository that back
runtime-path graph reads with real persistence. Graph.Api Program.cs wires
the new repository into the DI graph. InMemory* services get small cleanups
so they remain viable for tests and local dev.

CompatibilityEndpoints: extends the integration-test surface.

Tests: GraphPostgresRuntimeIntegrationTests,
GraphRuntimeRepositoryRegistrationTests, expanded
GraphCompatibilityEndpointsIntegrationTests.

Docs: graph architecture page updated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/modules/graph/architecture.md

@@ -31,6 +31,7 @@
 - `POST /graph/diff` — compares `snapshotA` vs `snapshotB`, streaming node/edge added/removed/changed tiles plus stats; budget enforcement mirrors `/graph/query`.
 - `POST /graph/export` — async job producing deterministic manifests (`sha256`, size, format) for `ndjson/csv/graphml/png/svg`; download via `/graph/export/{jobId}`.
 - `POST /graph/lineage` - returns SBOM lineage nodes/edges anchored by `artifactDigest` or `sbomDigest`, with optional relationship filters and depth limits.
+- Runtime repository selection is config-driven at service resolution time: when `Postgres:Graph` is configured, the live `/graph/query`, `/graph/diff`, and shipped `/graphs*` compatibility surfaces materialize persisted rows from `graph.graph_nodes`, `graph.graph_edges`, and `graph.pending_snapshots`; when it is not configured, the runtime fallback is an empty in-memory repository rather than historical demo-seeded graph data.
 - Compatibility facade for the shipped Angular explorer:
   - `GET /graphs`, `GET /graphs/{graphId}`, `GET /graphs/{graphId}/tiles`
   - `GET /search`, `GET /paths`