feat(admin): runtime-editable crawler & analysis config in the dashboard

Move crawler and analysis configuration out of boot-only env vars and into
a DB-backed, admin-editable surface applied live without a restart.

- New `app_settings` table (migration 0026) holds one JSONB row per group;
  env vars seed it on first boot, then the DB is the source of truth.
- `settings.rs` adds serializable Crawler/Analysis DTOs with env-base +
  DB-overlay conversion and field-level validation; env-only/secret fields
  (browser, proxy, TOR, vision API key, PHPSESSID) are never persisted.
- `GET/PUT /api/v1/admin/settings/{crawler,analysis}` (RequireAdmin,
  cookie-only, CSRF-guarded): GET returns the editable DTO + a read-only
  env-managed view + prompt defaults; PUT validates (422 + per-field
  details), persists + audits in one tx, then gracefully respawns the
  affected daemon via a Supervisor.
- AppState swaps the crawler control/resync handles and analysis enable
  gate behind shared runtime cells so reloads take effect with no restart;
  a boot-time spawn failure is logged rather than aborting startup.
- Make the three vision prompts and sampling temperature configurable
  (defaults preserved); cookie_domain is admin-editable too.
- Frontend: tabbed /admin/settings page with grouped fieldsets, prompt
  reset-to-default, env-managed read-only panel, save-&-apply confirm, and
  per-field validation; typed client + ApiError.details.
- Bump version 0.79.1 -> 0.80.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/api/chapters.rs

@@ -161,7 +161,7 @@ async fn create(
     // rolled-back upload never leaves jobs pointing at nonexistent pages; a
     // failed enqueue is logged but doesn't fail the upload (the admin
     // re-enqueue endpoint can backfill).
-    if state.analysis_enabled {
+    if state.analysis_enabled() {
         for page_id in page_ids {
             if let Err(e) =
                 repo::page_analysis::enqueue_for_page(&state.db, page_id, false).await