Add last login tracking, batch date auto-fill, and bug fixes

- Track last_login_at on User model, updated on every successful login
- Show last login date in admin panel user table
- Fix admin/garden date display (datetime strings already contain T separator)
- Fix My Garden Internal Server Error (MySQL does not support NULLS LAST syntax)
- Fix Log Batch infinite loop when user has zero varieties
- Auto-fill batch dates from today when creating a new batch, calculated
  from selected variety's week offsets (germination, greenhouse, garden)
- Update README with new features and batch date auto-fill formula table

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

backend/models.py

@@ -44,6 +44,7 @@ class User(Base):
     is_admin = Column(Boolean, default=False, nullable=False)
     is_disabled = Column(Boolean, default=False, nullable=False)
     created_at = Column(DateTime, server_default=func.now())
+    last_login_at = Column(DateTime, nullable=True)
 
     varieties = relationship("Variety", back_populates="user", cascade="all, delete-orphan")
     batches = relationship("Batch", back_populates="user", cascade="all, delete-orphan")

backend/routers/admin.py

@@ -23,7 +23,7 @@ def list_users(db: Session = Depends(get_db), _: User = Depends(get_admin_user))
     return [
         AdminUserOut(
             id=u.id, email=u.email, is_admin=u.is_admin, is_disabled=u.is_disabled,
-            created_at=u.created_at, **_user_stats(db, u.id)
+            created_at=u.created_at, last_login_at=u.last_login_at, **_user_stats(db, u.id)
         )
         for u in users
     ]

backend/routers/auth.py

@@ -1,3 +1,5 @@
+from datetime import datetime, timezone
+
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.orm import Session
 
@@ -48,6 +50,8 @@ def login(data: UserLogin, db: Session = Depends(get_db)):
         raise HTTPException(status_code=401, detail="Invalid email or password")
     if user.is_disabled:
         raise HTTPException(status_code=403, detail="Account has been disabled")
+    user.last_login_at = datetime.now(timezone.utc)
+    db.commit()
     return {"access_token": create_access_token(user.id), "token_type": "bearer"}
 
 

backend/routers/batches.py

@@ -1,4 +1,5 @@
 from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy import case
 from sqlalchemy.orm import Session, joinedload
 from typing import List
 
@@ -16,7 +17,7 @@ def list_batches(db: Session = Depends(get_db), current_user: User = Depends(get
         db.query(Batch)
         .options(joinedload(Batch.variety))
         .filter(Batch.user_id == current_user.id)
-        .order_by(Batch.sow_date.desc().nullslast(), Batch.created_at.desc())
+        .order_by(case((Batch.sow_date.is_(None), 1), else_=0), Batch.sow_date.desc(), Batch.created_at.desc())
         .all()
     )
 

backend/schemas.py

@@ -33,6 +33,7 @@ class AdminUserOut(BaseModel):
     is_admin: bool
     is_disabled: bool
     created_at: Optional[datetime]
+    last_login_at: Optional[datetime]
     variety_count: int
     batch_count: int