chore: remove pre-pivot dead code + finish Scribe rebrand (#599 t1-3)

- Header wordmark Fabled -> Scribe; fable:calendar-changed event ->
  scribe:calendar-changed; SettingsView CSS comment.
- Drop dead Project.auto_summary + summary_updated_at columns (migration
  0063) -- the Ollama-era summarizer is gone; model + 2 frontend types +
  projects test updated.
- Remove pivot vestiges: diagnostics _curator_busy()/curator_busy
  heartbeat field, tz BRIEFING_DAY_START_HOUR/user_briefing_date dead
  aliases, the ignored 'model' param on get_embedding (+ its test).

ruff src/ clean; CI is the gate. Part of scribe plan #599.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/AppHeader.vue

@@ -39,7 +39,7 @@ router.afterEach(() => {
       <!-- Left: brand -->
       <router-link to="/" class="nav-brand">
         <AppLogo :size="34" />
-        <span class="brand-text">Fabled</span>
+        <span class="brand-text">Scribe</span>
       </router-link>
 
       <!-- Center: primary navigation (desktop) -->