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/schemas/user.py

@@ -12,6 +12,7 @@ class UserBase(BaseModel):
 class UserCreate(UserBase):
     password: str = Field(..., min_length=10, description="Password must be at least 10 characters")
     role: str = Field(default="engineer", description="User role: admin, engineer, or viewer")
+    invite_code: Optional[str] = Field(None, description="Invite code for registration (required when invite system is enabled)")
 
 
 class UserUpdate(BaseModel):