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

tests/test_notes.py

@@ -2,7 +2,13 @@ import pytest
 
 from thoughtsync.app import create_app
 from thoughtsync.models.note import NOTE_COLORS, Note
-from thoughtsync.notes import is_empty_note, normalize_color, parse_link_titles, rewrite_link_title
+from thoughtsync.notes import (
+    derive_display_title,
+    is_empty_note,
+    normalize_color,
+    parse_link_titles,
+    rewrite_link_title,
+)
 
 
 @pytest.fixture
@@ -97,6 +103,32 @@ def test_rewrite_link_title_noop():
     assert rewrite_link_title("no links here", "alpha", "Gamma") == "no links here"
 
 
+def test_derive_display_title_explicit_wins():
+    assert derive_display_title("My Title", "some body line") == "My Title"
+    assert derive_display_title("  Padded  ", "body") == "Padded"
+
+
+def test_derive_display_title_from_first_body_line():
+    assert derive_display_title(None, "first line\nsecond line") == "first line"
+    assert derive_display_title("", "  spaced first  \nnext") == "spaced first"
+    # leading blank/whitespace lines are skipped to the first line with content
+    assert derive_display_title(None, "\n   \nreal line\nmore") == "real line"
+    # a whitespace-only title falls through to the body
+    assert derive_display_title("   ", "body wins") == "body wins"
+
+
+def test_derive_display_title_empty():
+    assert derive_display_title(None, None) == ""
+    assert derive_display_title("", "") == ""
+    assert derive_display_title("   ", "   \n  ") == ""
+
+
+def test_derive_display_title_caps_length():
+    long = "x" * 300
+    assert derive_display_title(None, long) == "x" * 200
+    assert derive_display_title(long, "body") == "x" * 200
+
+
 async def test_titles_requires_auth(app):
     client = app.test_client()
     resp = await client.get("/api/notes/titles")