feat: add nightly session notes

[kilo-WATT]

Summary

Adds simple plain-text notes for SleepLab session/night detail pages.

Users can now add, edit, view, and clear an optional note for a sleep session/night. Notes are stored on the existing sessions table with a nullable note column.

Because SleepLab already treats user_id + folder_date as the nightly boundary, saving a note updates all session rows for that same user/night so multi-block nights stay consistent.

Changes

• Added migration 018_add_session_note.sql
• Extended session detail API response with note
• Added PUT /sessions/{session_id}/note
• Added a compact Notes section to SessionDetail.tsx
• Added backend tests for note persistence, updating, clearing, and session detail response

Screenshot

Notes section on the session detail page:

Validation

• Migration applied successfully in local LXC deployment
• App rebuilt and started healthy
• Manual UI test passed: add note, save, refresh, edit, clear
• uv run python scripts\check_migrations.py passed
• uv run pytest -q passed available tests: 37 passed, 74 skipped
• npm run test in frontend passed: 6 passed
• npm run build in frontend passed
• npm run lint still has pre-existing repo-wide lint failures unrelated to this PR