Implement identity graph stitching and configurable profile storage

Issue persistent signed anonymous IDs and accept cookie/token-bound events.
Resolve verified email, phone, OAuth, and namespaced third-party IDs into
transactionally merged master identities with historical event aliases.
Protect shared-device account switching, tenant isolation, and event retries.

Store sparse canonical profiles as PostgreSQL JSONB or local SQLite JSON.
Support an optional versioned Cassandra wide-column projection through a
transactional outbox, retry worker, and immediate canonical read fallback.
Document deployment settings and provide opt-in database integration tests.

Validation: 43 local tests passed; 6 external database tests skipped.
Ruff checks, formatting, and offline uv lock verification passed.

.gitignore

@@ -9,3 +9,7 @@ dist/
 .env
 .env.*
 !.env.example
+.data/
+*.db
+*.db-shm
+*.db-wal