M6 1901: URL capture with link-preview unfurl (SSRF-hardened)

Paste a link → fetch its OpenGraph/meta preview (title, description, image,
site) and show a rich card. User-triggered + persisted (never auto-fetches;
cached so it never re-fetches). Opt-in via a new admin setting
enable_url_unfurl (default on, rule 26).

Security (the whole point of this task): a new dependency-free unfurl.py
does the fetch with layered SSRF defenses — http/https only; resolve the
host and reject EVERY non-public address (private/loopback/link-local/
reserved/multicast/unspecified — blocks 169.254.169.254 etc.); connect to
the vetted IP with SNI so DNS-rebinding can't slip through; ≤3 redirects
each re-validated; 5s timeout; 512 KB cap; text/html only; blocking IO in a
worker thread. No server-side image fetch — the og:image URL is loaded by
the browser.

- note_link_previews table (migration 0020), one per (note, url); serialized
  inline on notes (+ rides the sync pull feed read-only).
- POST /api/notes/<id>/unfurl {url} (owner-scoped, setting-gated, 502 on
  fetch failure); DELETE /api/notes/<id>/previews/<id>.
- enable_url_unfurl exposed in public config so the UI hides the affordance
  when disabled.

Frontend: LinkPreview.vue card; editor detects URLs in the body and offers a
"Preview <domain>" chip per un-previewed link (ensureDraft first), renders
preview cards with remove; card shows previews read-only. New link icon;
notes-store unfurl()/deletePreview().

Tests (DB-free): is_public_ip range blocking, validate_url scheme/parts,
extract_preview (OG + <title> fallback + relative-image resolve), endpoint
auth-guards.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/NoteCard.vue

@@ -11,6 +11,7 @@ import {
 } from "../notes/colors";
 import type { Note } from "../stores/notes";
 import Icon from "./Icon.vue";
+import LinkPreview from "./LinkPreview.vue";
 import MarkdownText from "./MarkdownText.vue";
 import NoteChecklist from "./NoteChecklist.vue";
 import { formatReminder, isOverdue } from "../notes/datetime";
@@ -162,6 +163,10 @@ onBeforeUnmount(() => document.removeEventListener("mousedown", onDocMousedown))
       </span>
     </div>
 
+    <div v-if="note.previews.length" class="mb-2 flex flex-col gap-2">
+      <LinkPreview v-for="p in note.previews" :key="p.id" :preview="p" />
+    </div>
+
     <!-- Checklist notes can't nest interactive controls in a <button>, so use a
          focusable div; text notes keep a semantic button. -->
     <template v-if="note.kind === 'list'">