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>

web/templates/pages/fave_detail.html

@@ -2,7 +2,13 @@
 {{with .Data}}{{with .Fave}}
     {{if eq .Privacy "public"}}
         <meta property="og:title" content="{{truncate 70 .Description}}">
-        <meta property="og:description" content="En favoritt av {{.DisplayName}} på {{$.SiteName}}">
+        {{if .Notes}}
+            <meta property="og:description" content="{{truncate 200 .Notes}}">
+        {{else if .URL}}
+            <meta property="og:description" content="{{.URL}}">
+        {{else}}
+            <meta property="og:description" content="En favoritt av {{.DisplayName}} på {{$.SiteName}}">
+        {{end}}
         <meta property="og:type" content="article">
         {{if $.ExternalURL}}
             <meta property="og:url" content="{{$.ExternalURL}}/faves/{{.ID}}">
@@ -45,6 +51,10 @@
                 <p><a href="{{.URL}}" target="_blank" rel="noopener noreferrer">{{.URL}}</a></p>
             {{end}}
 
+            {{if .Notes}}
+                <div class="fave-notes">{{.Notes}}</div>
+            {{end}}
+
             {{if .Tags}}
                 <p>
                     {{range .Tags}}

web/templates/pages/fave_form.html

@@ -30,6 +30,13 @@
                        placeholder="https://...">
             </label>
 
+            <label for="notes">
+                Notater (valgfri)
+                <textarea id="notes" name="notes"
+                          rows="4"
+                          placeholder="Utfyllende tekst, anmeldelse, tanker...">{{.Notes}}</textarea>
+            </label>
+
             <label for="image">
                 Bilde (valgfri)
                 <input type="file" id="image" name="image"