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/views/SettingsView.vue

@@ -3076,7 +3076,7 @@ function formatUserDate(iso: string): string {
 .settings-empty { opacity: 0.5; margin-top: 1rem; }
 .settings-description { opacity: 0.7; margin-bottom: 1rem; line-height: 1.5; }
 
-/* Fable MCP section */
+/* Scribe MCP section */
 .mcp-status { opacity: 0.6; font-size: 0.9rem; }
 .mcp-unavailable p { opacity: 0.7; }
 .mcp-available { display: flex; flex-direction: column; gap: 1.25rem; }