Fix login blocked by registration password validation

Split UserCreate into UserCreate (with min_length constraints) and
UserLogin (no constraints) so existing short passwords can still log in.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/backend/app/routes/users.py

@@ -14,6 +14,11 @@ class UserCreate(BaseModel):
     password: str = Field(min_length=8)
 
 
+class UserLogin(BaseModel):
+    username: str
+    password: str
+
+
 class Token(BaseModel):
     access_token: str
     token_type: str
@@ -37,7 +42,7 @@ def register(user_in: UserCreate, db: Session = Depends(get_db)):
 
 
 @router.post("/login", response_model=Token)
-def login(user_in: UserCreate, db: Session = Depends(get_db)):
+def login(user_in: UserLogin, db: Session = Depends(get_db)):
     user = db.query(models.User).filter(models.User.username == user_in.username).first()
     if not user or not verify_password(user_in.password, user.password):
         raise HTTPException(status_code=401, detail="Invalid credentials")