add habit goal-period difficulty and habit formation research notes

Ground the 21-day habit myth in actual research (Lally et al. 2010) and let
users pick a target period (21/66/254 days or unlimited) matching that
study's easy/median/hard automaticity timelines when creating a habit,
with progress shown on the habits list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/routers/pages.py

@@ -9,12 +9,12 @@ from pydantic import ValidationError
 from sqlalchemy.orm import Session
 
 from app.database import get_db
-from app.models.habit import HabitStatus, HabitType
+from app.models.habit import HabitDifficulty, HabitStatus, HabitType
 from app.models.user import User
 from app.schemas.habit import HabitCreate, HabitUpdate
 from app.security import get_current_user_optional
 from app.services import habit_service, log_service
-from app.template_utils import heatmap_opacity, is_milestone_streak, weekday_label
+from app.template_utils import difficulty_label, goal_progress, heatmap_opacity, is_milestone_streak, weekday_label
 
 router = APIRouter()
 
@@ -22,6 +22,9 @@ templates = Jinja2Templates(directory="app/templates")
 templates.env.globals["weekday_label"] = weekday_label
 templates.env.globals["heatmap_opacity"] = heatmap_opacity
 templates.env.globals["is_milestone_streak"] = is_milestone_streak
+templates.env.globals["difficulty_label"] = difficulty_label
+templates.env.globals["goal_progress"] = goal_progress
+templates.env.globals["habit_difficulties"] = list(HabitDifficulty)
 
 
 def _current_user_or_redirect(request: Request, db: Session) -> User | RedirectResponse:
@@ -155,6 +158,7 @@ def create_habit_page(
     name: str = Form(""),
     habit_type: str = Form(...),
     weekdays_mask: int = Form(...),
+    difficulty: str = Form(HabitDifficulty.MEDIUM.value),
     condition_text: str | None = Form(None),
     reminder_time: str | None = Form(None),
     db: Session = Depends(get_db),
@@ -169,6 +173,7 @@ def create_habit_page(
             name=name,
             habit_type=HabitType(habit_type),
             weekdays_mask=weekdays_mask,
+            difficulty=HabitDifficulty(difficulty),
             condition_text=condition_text,
             reminder_time=parsed_time,
         )
@@ -190,6 +195,7 @@ def edit_habit_page(
     habit_id: int,
     name: str = Form(""),
     weekdays_mask: int = Form(...),
+    difficulty: str = Form(HabitDifficulty.MEDIUM.value),
     condition_text: str | None = Form(None),
     reminder_time: str | None = Form(None),
     db: Session = Depends(get_db),
@@ -208,6 +214,7 @@ def edit_habit_page(
             name=name,
             habit_type=habit.habit_type,
             weekdays_mask=weekdays_mask,
+            difficulty=HabitDifficulty(difficulty),
             condition_text=condition_text,
             reminder_time=parsed_time,
         )