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>

migrations/versions/0007_add_habit_difficulty.py

@@ -0,0 +1,31 @@
+"""add difficulty (target period) to habit
+
+Revision ID: 0007_add_habit_difficulty
+Revises: 0006_add_habit_abandoned
+Create Date: 2026-07-17
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0007_add_habit_difficulty"
+down_revision: Union[str, None] = "0006_add_habit_abandoned"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "habit",
+        sa.Column("difficulty", sa.String(length=20), nullable=False, server_default="medium"),
+    )
+    op.create_check_constraint(
+        "ck_habit_difficulty", "habit", "difficulty in ('easy','medium','hard','unlimited')"
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("ck_habit_difficulty", "habit", type_="check")
+    op.drop_column("habit", "difficulty")