Add invite code registration system for beta

Backend:
- Add InviteCode model with single-use codes
- Add invite API endpoints (create, list, revoke, validate)
- Modify registration to require invite code when enabled
- Add REQUIRE_INVITE_CODE config toggle (default: true)
- Add Alembic migration for invite_codes table

Frontend:
- Add invite code field to registration page
- Validate invite code on blur with visual feedback
- Pass invite code to registration API

Admins can generate invite codes via /api/docs (Swagger UI).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/api/endpoints/auth.py

@@ -5,6 +5,7 @@ from fastapi.security import OAuth2PasswordRequestForm
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
 
+from app.core.config import settings
 from app.core.database import get_db
 from app.core.security import (
     verify_password,
@@ -14,6 +15,7 @@ from app.core.security import (
     decode_token
 )
 from app.models.user import User
+from app.models.invite_code import InviteCode
 from app.schemas.user import UserCreate, UserResponse, UserLogin
 from app.schemas.token import Token
 from app.api.deps import get_current_user
@@ -27,6 +29,39 @@ async def register(
     db: Annotated[AsyncSession, Depends(get_db)]
 ):
     """Register a new user."""
+    # Validate invite code if required
+    invite_code_record = None
+    if settings.REQUIRE_INVITE_CODE:
+        if not user_data.invite_code:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Invite code is required"
+            )
+
+        # Look up invite code (case-insensitive)
+        result = await db.execute(
+            select(InviteCode).where(InviteCode.code == user_data.invite_code.upper())
+        )
+        invite_code_record = result.scalar_one_or_none()
+
+        if not invite_code_record:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Invalid invite code"
+            )
+
+        if invite_code_record.is_used:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Invite code has already been used"
+            )
+
+        if invite_code_record.is_expired:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Invite code has expired"
+            )
+
     # Check if email already exists
     result = await db.execute(select(User).where(User.email == user_data.email))
     existing_user = result.scalar_one_or_none()
@@ -41,9 +76,16 @@ async def register(
         email=user_data.email,
         password_hash=get_password_hash(user_data.password),
         name=user_data.name,
-        role=user_data.role  # Use role from request (defaults to "engineer")
+        role=user_data.role,
+        invite_code_id=invite_code_record.id if invite_code_record else None
     )
     db.add(new_user)
+
+    # Mark invite code as used
+    if invite_code_record:
+        invite_code_record.used_by_id = new_user.id
+        invite_code_record.used_at = datetime.now(timezone.utc)
+
     await db.commit()
     await db.refresh(new_user)