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.

deploy/cassandra.cql

@@ -0,0 +1,14 @@
+-- Example for a single datacenter named datacenter1; set production RF appropriately.
+CREATE KEYSPACE IF NOT EXISTS adcloud
+WITH replication = {'class': 'NetworkTopologyStrategy', 'datacenter1': 1};
+
+-- Each immutable profile revision is a separate bounded partition.
+-- Sparse attributes are individual fields; absent attributes occupy no rows.
+CREATE TABLE IF NOT EXISTS adcloud.identity_profile_fields (
+    tenant_id text,
+    master_id text,
+    revision int,
+    field text,
+    value text,
+    PRIMARY KEY ((tenant_id, master_id, revision), field)
+);