refactor(config): mandatory app-config rows seeded in DB; rename to session.setup_timeout

All registry keys must exist as rows in the config table: main seeds
missing keys with their defaults at startup, LoadApp fails fast on a
missing key, and the code-side fallback const/helper for the onboarding
setup timeout is gone -- the value rides in SessionConfig.SetupTimeout.
The key is renamed session.idle_timeout -> session.setup_timeout, and
the /config page's 'overridden' now means 'differs from the default'.

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

docs/superpowers/plans/2026-07-26-profile-deletion.md

@@ -508,7 +508,7 @@ to:
 				})
 ```
 
-- [ ] **Step 5: Add `handleDeleteProfile` to `profile.go`**
+- [ ] **Step 5: Add `handleDeleteProfile` to `profiles.go`**
 
 Append to `backend/internal/api/profile.go`:
 
@@ -554,7 +554,7 @@ Expected: `gofmt -l .` prints nothing; `go build`/`go vet`/`go test` all succeed
 - [ ] **Step 8: Commit**
 
 ```bash
-git add backend/internal/api/server.go backend/internal/api/profile.go backend/internal/api/api_test.go backend/internal/api/isolation_test.go
+git add backend/internal/api/server.go backend/internal/api/profiles.go backend/internal/api/api_test.go backend/internal/api/isolation_test.go
 git commit -m "feat(api): add DELETE /api/profile with Garmin client/tokenstore teardown"
 ```