test(api): pin the resume-rewind contract

TestAudioResumeRewind drives the whole surface against the shared dev
DB with the fresh per-test regular user (the live admin is never
touched; the fixture book lives in a test-named library that cascades
away): default 0, profile PUT "5" rides the chapters payload,
non-allow-listed values 400 without side effects, explicit "0"
resets everywhere, and an omitted JSON key leaves the column
untouched.

Changelog entry in docs/contract_pins.md (additive chapters key +
the profile field's string-shaped PUT, keep_screen_awake precedent)
and the get_profile.md example gains the preference keys.

Debug note for the next agent: the first runs here 401'd with
'Ghost session rejected' — NOT a harness or auth bug. The tests never
execute schema.sql; the shared dev DB gets its schema from the app
container's boot. The rebuilt tests image ran the new GetUser
(selecting audio_resume_rewind_seconds) against a DB whose schema
came from the old app image — every GetUser errored, and the
ghost-session guard turned that into 401s. Applying the schema.sql
ALTER to the dev DB (what the next app rebuild does anyway) turned
the suite green. Until make rebuild-app runs, the running app
container serves the old binary.

docs/developer/api/users/get_profile.md

@@ -36,6 +36,8 @@ Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
   "first_name": "John",
   "last_name": "Doe",
   "role": "user",
+  "default_audio_speed": 1.5,
+  "audio_resume_rewind_seconds": 5,
   "keep_screen_awake": true
 }
 ```