feat(billing): add INTERNAL_TESTER_EMAILS allowlist for self-serve soft cutover

Phase O Task 46 needs internal validation of the full self-serve flow
against the prod backend before flipping SELF_SERVE_ENABLED public. This
adds the per-email allowlist that bypasses the global flag for specific
authenticated users.

- INTERNAL_TESTER_EMAILS: comma-separated list, parsed by a Pydantic
  field_validator into a normalized lowercase list. Settings.is_internal_tester
  and Settings.is_self_serve_active_for centralize the allowlist + global-flag
  check; both endpoints below call the latter.
- New get_current_user_optional dep — best-effort auth that returns None
  on missing/invalid token instead of 401. Used by /config/public so the
  same endpoint serves anonymous public callers and authenticated allowlist
  members.
- /config/public now accepts optional auth and returns self_serve_enabled=True
  for authenticated allowlist members even when the global flag is off.
  Anonymous callers always see the global flag.
- /auth/register replaces the SELF_SERVE_ENABLED check with the helper so a
  registering email on the allowlist can join without an invite code.
  Non-allowlist emails still 400 when self-serve is off.
- docker-compose.dev.yml passes SELF_SERVE_ENABLED + INTERNAL_TESTER_EMAILS
  through; backend/.env.example documents both.

Tests cover: allowlisted authenticated user sees true, non-allowlisted
authenticated user sees the global flag, anonymous calls ignore the
allowlist, allowlisted email registers without invite code, non-allowlisted
email still blocked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/deps.py

@@ -64,6 +64,40 @@ async def get_current_user(
     return user
 
 
+async def get_current_user_optional(
+    request: Request,
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
+) -> Optional[User]:
+    """Best-effort current user for endpoints that work both anonymous and authed.
+
+    Returns None on missing/invalid/expired token instead of raising. Used by
+    surfaces like /config/public that anonymous clients can hit but where an
+    authenticated user gets a tailored response (e.g. INTERNAL_TESTER_EMAILS
+    allowlist override).
+    """
+    auth_header = request.headers.get("Authorization") or request.headers.get("authorization")
+    if not auth_header or not auth_header.lower().startswith("bearer "):
+        return None
+    token = auth_header.split(None, 1)[1].strip()
+    if not token:
+        return None
+
+    payload = decode_token(token)
+    if payload is None or payload.get("type") != "access":
+        return None
+
+    user_id = payload.get("sub")
+    if user_id is None:
+        return None
+    try:
+        user_uuid = UUID(user_id)
+    except ValueError:
+        return None
+
+    result = await db.execute(select(User).where(User.id == user_uuid))
+    return result.scalar_one_or_none()
+
+
 async def get_refresh_token_payload(
     token: Annotated[str, Depends(oauth2_scheme)]
 ) -> dict: