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/nginx.conf.example

@@ -28,6 +28,11 @@ server {
     proxy_send_timeout 30s;
 
     # No URI suffix on proxy_pass: preserve /v1/... and query strings.
+    # Exact roots prevent nginx from redirecting POST /v1/events to a trailing slash.
+    location = /v1/events { proxy_pass http://adcloud_events; }
+    location = /v1/users { proxy_pass http://adcloud_users; }
+    location = /v1/ads { proxy_pass http://adcloud_ads; }
+    location = /v1/identity { proxy_pass http://adcloud_identity; }
     location ^~ /v1/events/ { proxy_pass http://adcloud_events; }
     location ^~ /v1/users/ { proxy_pass http://adcloud_users; }
     location ^~ /v1/ads/ { proxy_pass http://adcloud_ads; }