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

src/thoughtsync/settings.py

@@ -51,6 +51,15 @@ REGISTRY: list[SettingDef] = [
         "Largest single file that can be attached to a note. Capped by the server body limit.",
         "Attachments",
     ),
+    SettingDef(
+        "enable_url_unfurl",
+        "bool",
+        True,
+        "Link previews",
+        "Let the server fetch a page's title/description/image to preview pasted links. "
+        "The server contacts the linked site; private/internal addresses are always blocked.",
+        "Links",
+    ),
 ]
 
 _BY_KEY: dict[str, SettingDef] = {d.key: d for d in REGISTRY}
@@ -112,6 +121,7 @@ async def get_public_config(db) -> dict:
     return {
         "site_name": await get_setting(db, "site_name"),
         "allow_registration": await get_setting(db, "allow_registration"),
+        "enable_url_unfurl": await get_setting(db, "enable_url_unfurl"),
     }