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/app/routers/auth.py

@@ -21,6 +21,9 @@ pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
 
 router = APIRouter()
 
+## Minimum age to register. This platform is adults-only.
+MIN_AGE = 18
+
 
 # --- Pydantic schemas ---
 
@@ -39,14 +42,19 @@ class RegisterRequest(BaseModel):
     username: str
     email: EmailStr
     password: str
-    over_18: bool  # Simple boolean confirmation instead of date_of_birth
+    # Real date of birth -- the server computes age from it and rejects
+    # anyone under MIN_AGE. Still self-attested (no document check yet), but
+    # a concrete, stored, legally-meaningful data point rather than a
+    # throwaway checkbox, and the seam for a future hard ID-verification
+    # pass. See docs/architecture/content_moderation.md 5.2.
+    date_of_birth: date
 
 
 class UserResponse(BaseModel):
     id: int
     username: str
     email: str
-    date_of_birth: str | None = None
+    date_of_birth: date | None = None
     is_active: bool
     created_at: datetime | None = None
 
@@ -108,15 +116,9 @@ def hash_password(password: str) -> str:
 def calculate_age(birth_date: date, today: date | None = None) -> int:
     """Calculate age in years from a birth date.
 
-    Not called by /register anymore (that endpoint now just trusts the
-    client's over_18 boolean, see RegisterRequest) -- kept as a real,
-    tested utility since date-of-birth storage/verification is coming
-    back in a future pass (real age math, not just a client-side claim,
-    the user explicitly wants to revisit this design). Removing it broke
-    backend/tests/test_auth_age_gate.py's collection entirely (ImportError
-    on `from app.routers.auth import calculate_age`) for zero benefit --
-    restoring it is the lower-risk path over rewriting or deleting 17
-    tests for logic this codebase will need again.
+    Used by /register (PC3 Phase 3) to compute age from the submitted
+    date_of_birth and enforce the 18+ gate. See
+    docs/architecture/content_moderation.md 5.2.
 
     Args:
         birth_date: The person's date of birth.
@@ -155,12 +157,20 @@ def _issue_refresh_token(user_id: int, db: AsyncSession) -> str:
 async def register(req: RegisterRequest, db: AsyncSession = Depends(get_db)):
     """Register a new user and return access + refresh tokens.
 
-    Requires over_18 boolean confirmation; rejects users who are not 18+.
+    Requires a date of birth; the server computes age and rejects anyone
+    under MIN_AGE (18). See docs/architecture/content_moderation.md for the
+    verification design and its self-attestation limitation.
     """
-    if not req.over_18:
+    age = calculate_age(req.date_of_birth)
+    if age < 0 or req.date_of_birth > date.today():
         raise HTTPException(
             status_code=status.HTTP_400_BAD_REQUEST,
-            detail="Must be 18 or older to register",
+            detail="Invalid date of birth",
+        )
+    if age < MIN_AGE:
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail=f"You must be at least {MIN_AGE} to use this platform",
         )
 
     # Check if username already exists
@@ -183,7 +193,7 @@ async def register(req: RegisterRequest, db: AsyncSession = Depends(get_db)):
         username=req.username,
         email=req.email,
         hashed_password=hash_password(req.password),
-        date_of_birth=None,  # No longer collecting DOB
+        date_of_birth=req.date_of_birth,
     )
     db.add(user)
     await db.flush()  # flush to get user.id before creating refresh token