feat: add Atom feeds and JSON/CSV import/export

Phase 5 — Feeds & Import/Export:
- Atom feeds: global (/feed.xml), per-user (/u/{name}/feed.xml),
  per-tag (/tags/{name}/feed.xml). Uses gorilla/feeds.
- JSON export: all user's faves with tags, pretty-printed
- CSV export: standard format with header row
- JSON import: validates and creates faves with tags
- CSV import: flexible column mapping from header row
- Import/export pages with format documentation
- Feed items include enclosure for images, author info
- Limited-visibility profiles excluded from feeds

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

web/templates/pages/import.html

@@ -0,0 +1,32 @@
+{{define "content"}}
+<h1>Importer favoritter</h1>
+
+<article>
+    <p>Last opp en JSON- eller CSV-fil med favoritter. Bilder støttes ikke ved import.</p>
+    <form method="POST" action="{{basePath}}/import" enctype="multipart/form-data">
+        <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
+        <label for="file">
+            Velg fil (JSON eller CSV)
+            <input type="file" id="file" name="file" required
+                   accept=".json,.csv,application/json,text/csv">
+        </label>
+        <button type="submit">Importer</button>
+    </form>
+
+    <details>
+        <summary>Forventet format</summary>
+        <h3>JSON</h3>
+        <pre><code>[
+  {
+    "description": "Blade Runner 2049",
+    "url": "https://example.com",
+    "privacy": "public",
+    "tags": ["film", "sci-fi"]
+  }
+]</code></pre>
+        <h3>CSV</h3>
+        <pre><code>description,url,privacy,tags
+Blade Runner 2049,https://example.com,public,"film,sci-fi"</code></pre>
+    </details>
+</article>
+{{end}}