feat(career): gigs backend — propose a setlist, log the completed set

Career v3, WS3 (backend half). A gig is career's verb:

- POST /gigs/propose {instrument, genre, size}: setlist from the
  passport's own stubs — qualifying songs (per the genre's badge bar,
  family-aware) shuffled for a free re-roll, topped with the
  highest-accuracy near-bar songs as stakes, and filled from UNPLAYED
  genre songs when the passport is young (the first gig is how stubs
  start). Names the highest venue the current stars can book.
- POST /gigs: logs a COMPLETED set only (abandoned sets never log — no
  fail state). Per-song accuracy = MAX(last_accuracy) from song_stats,
  freshly written by the set's own plays; encore = avg ≥ the data-driven
  bar (passports.json gig.encore_accuracy, 0.75). Appends to the career
  state file (same atomic _save_json pattern).
- Passports view: per-passport gigs (newest first, capped 20) and
  per-instrument gig_count — the profile wall's gig line lights up.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -19,6 +19,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   copied straight to the clipboard for pasting outside the app (shared
   `blob-io` helpers replace the download idiom previously duplicated in
   settings-io and diagnostics-export).
+- **Gigs (backend)** — career mode gains its verb: `POST
+  /api/plugins/career/gigs/propose` builds a playable setlist for an
+  instrument+genre (your qualifying songs plus a couple of stakes songs near
+  the bar; a young passport fills from unplayed genre songs — the first gig
+  is how stubs start; re-roll by calling again), naming the room your stars
+  can book. `POST /gigs` logs a **completed** set — per-song accuracies read
+  from the set's own freshly-recorded stats, an encore flag at the
+  data-driven bar (avg ≥ 75%) — into the career state; abandoned sets never
+  log (no fail state: the gig you finished is the gig you played). Passports
+  carry their gig log; instruments their gig count.
 
 ### Changed
 - **The full mix is a stem** (#933) — core no longer depends on `original_audio:`, a