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/models/all.py

@@ -11,6 +11,7 @@ from . import (  # noqa: F401
     note_attachment,
     note_item,
     note_link,
+    note_link_preview,
     note_revision,
     settings,
     share,

src/thoughtsync/models/note_link_preview.py

@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, Text, UniqueConstraint, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from . import Base
+
+
+class NoteLinkPreview(Base):
+    """A cached OpenGraph/meta preview for a URL in a note, fetched server-side on the
+    user's request (see unfurl.py). Stored so it never re-fetches. One per (note, url)."""
+
+    __tablename__ = "note_link_previews"
+    __table_args__ = (UniqueConstraint("note_id", "url", name="uq_note_link_previews_note_url"),)
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    note_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("notes.id", ondelete="CASCADE"), nullable=False
+    )
+    url: Mapped[str] = mapped_column(Text(), nullable=False)
+    title: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    description: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    image_url: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    site_name: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())