m4.5: titles optional — every note gets an auto display name

Capture starts in the body, so forcing a title feels odd and body-only
notes had no name — which made them unlinkable. Fix both: persist a
display_title = explicit title if set, else the note's first non-empty
body line (deterministic, no AI). The title field stays optional.

- migration 0012: notes.display_title (NOT NULL, best-effort backfill;
  the app recomputes precisely on next save)
- derive_display_title() helper, set on create + update
- drive the /titles index, backlinks, graph edges + node labels, and
  [[wiki-link]] resolution off display_title so body-only notes are
  nameable, findable (command palette / [[ autocomplete), and linkable
- rename-repoint generalized: inbound [[Old Name]] links now survive a
  name change via the first body line too, not just an explicit title
- unit tests for the derivation (explicit wins, first non-empty line,
  blank/empty, length cap)
- frontend: display_title on the Note type; title field placeholder now
  reads "Title (optional)"

First item of M4.5 (frictionless input & recall); unblocks the linking
work. Card rendering unchanged (no first-line duplication).

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

src/thoughtsync/models/note.py

@@ -38,6 +38,11 @@ class Note(Base):
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"), nullable=False
     )
     title: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    # The note's display NAME: explicit title if set, else the first non-empty body
+    # line (see notes.derive_display_title). Persisted + normalized-matched so every
+    # note — even a body-only one — is nameable, searchable, graphable, and
+    # [[wiki-link]]-able without forcing the user to type a title.
+    display_title: Mapped[str] = mapped_column(Text(), nullable=False, server_default="")
     body: Mapped[str] = mapped_column(Text(), nullable=False, server_default="")
     color: Mapped[str] = mapped_column(Text(), nullable=False, server_default="default")
     # 'text' (freeform body) or 'list' (a checklist of note_items).
@@ -59,6 +64,7 @@ class Note(Base):
         return {
             "id": str(self.id),
             "title": self.title,
+            "display_title": self.display_title,
             "body": self.body,
             "color": self.color,
             "kind": self.kind,