PC3 Phase 3.2: real age verification (date of birth, not a checkbox)

POST /api/auth/register now requires a real date_of_birth (ISO YYYY-MM-DD).
The server computes age (auth.calculate_age) and rejects under-18s with 403;
a future/nonsensical date is 400. The DOB is stored and surfaced only to
that user (/api/auth/me).

Still self-attested (a user can enter a false DOB) -- but a concrete,
stored, legally-meaningful data point and the seam for a future ID-check
pass. Encryption-at-rest for the column and the Tier-2 hook are not done;
the limitation is stated plainly in docs/architecture/content_moderation.md 5.2.

- auth.py: RegisterRequest.date_of_birth (date); MIN_AGE=18; register()
  computes + enforces; UserResponse.date_of_birth typed `date | None`
  (the old `str | None` would have 500'd /me once a DOB was actually stored).
- api_client.gd / main_menu.gd: the Over18 checkbox is now a
  "Date of birth (YYYY-MM-DD)" field with a client-side sanity check.
- tests: test_auth_age_gate.py rewritten around the endpoint + age math
  (adult/minor/exactly-18/future-dob/stored); ~25 over_18 references across
  6 backend test files + 6 game smoke tests updated. Backend 74 passed;
  game smokes green (auth 6/6, menu_full_flow 12/12, e2e 4/4, etc.);
  fresh-clone boot PASS.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

backend/tests/test_world_join.py

@@ -13,7 +13,7 @@ from app.main import app
 
 def _auth(client: TestClient, name: str) -> tuple[dict, int]:
     r = client.post("/api/auth/register", json={
-        "username": name, "email": f"{name}@e.com", "password": "secret123", "over_18": True,
+        "username": name, "email": f"{name}@e.com", "password": "secret123", "date_of_birth": "1990-01-01",
     })
     assert r.status_code == 201, r.text
     headers = {"Authorization": f"Bearer {r.json()['access_token']}"}