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.

.env.example

@@ -1 +1,20 @@
+# Deployment profile selection. Canonical graph always uses DATABASE_URL.
+DATABASE_URL=sqlite:///.data/adcloud.db
+PROFILE_BACKEND=relational
+TENANT_ID=default
+# Generate once per deployment; keep stable across workers/restarts.
+IDENTITY_SECRET=
+# Trusted backend only; never ship this value to browser/SDK builds.
+SERVICE_TOKEN=
+COOKIE_SECURE=false
+COOKIE_SAMESITE=lax
+COOKIE_MAX_AGE=31536000
 CORS_ORIGINS=http://localhost:3000,http://localhost:5173,http://localhost:5174
+# Used only when PROFILE_BACKEND=cassandra.
+CASSANDRA_HOSTS=127.0.0.1
+CASSANDRA_PORT=9042
+CASSANDRA_KEYSPACE=adcloud
+CASSANDRA_USERNAME=
+CASSANDRA_PASSWORD=
+CASSANDRA_TLS=true
+CASSANDRA_CA_FILE=