feat: add notes field to favorites and enhance OG meta tags

Add an optional long-form "notes" text field to each favorite for
reviews, thoughts, or extended descriptions. The field is stored in
SQLite via a new migration (002_add_fave_notes.sql) and propagated
through the entire stack:

- Model: Notes field on Fave struct
- Store: All SQL queries (Create, GetByID, Update, list methods,
  scanFaves) updated with notes column
- Web handlers: Read/write notes in create, edit, update forms
- API handlers: Notes in create, update, get, import request/response
- Export: Notes included in both JSON and CSV exports
- Import: Notes parsed from both JSON and CSV imports
- Feed: Notes used as Atom feed item summary when present
- Form template: New textarea between URL and image fields
- Detail template: Display notes, enhanced og:description with
  cascade: notes (truncated) → URL → generic fallback text

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/database/migrations/002_add_fave_notes.sql

@@ -0,0 +1,2 @@
+-- Legger til et valgfritt notatfelt på favoritter.
+ALTER TABLE faves ADD COLUMN notes TEXT NOT NULL DEFAULT '';