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>
