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

alembic/versions/0012_note_display_title.py

@@ -0,0 +1,36 @@
+"""notes.display_title
+
+Revision ID: 0012
+Revises: 0011
+Create Date: 2026-07-22
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0012"
+down_revision = "0011"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # The note's display NAME: explicit title if set, else the first body line.
+    op.add_column("notes", sa.Column("display_title", sa.Text(), nullable=False, server_default=""))
+    # Best-effort backfill (kept intentionally simple + unambiguous — CI has no
+    # Postgres lane, so this only runs on a real deploy). The app recomputes the
+    # precise value via derive_display_title on the next save (which additionally
+    # skips leading blank lines). Capped at 200 chars.
+    op.execute(
+        r"""
+        UPDATE notes SET display_title = LEFT(btrim(
+            CASE
+                WHEN title IS NOT NULL AND btrim(title) <> '' THEN title
+                ELSE split_part(body, E'\n', 1)
+            END
+        ), 200)
+        """
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("notes", "display_title")