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

scripts/smoke_test_e2e.gd

@@ -69,7 +69,7 @@ func _run() -> void:
 
 	# --- Step 2: Register -----------------------------------------------------
 	print("\n=== Step 2: Register (POST /api/auth/register) ===")
-	var reg: Dictionary = await api.register(username, email, TEST_PASSWORD, true)
+	var reg: Dictionary = await api.register(username, email, TEST_PASSWORD, "1990-01-01")
 	var reg_token := _extract_token(reg)
 	var reg_ok: bool = (
 		reg.ok