fix(review-2): medium/low + docs — config validation, hygiene, doc sync

- compression_concurrency wired to boot config (state.rs) and removed as a
  dead admin control.
- patch_config gains per-key integer/range validation so numerically-valid-but-
  nonsensical values (-1/NaN/3.5) are rejected instead of silently reverting to
  default at read time.
- clearAuth now also clears the display name (shared-device residual).
- e2e/Caddyfile.test mirrors prod security headers + the SSE encode-exclusion so
  the test stack is representative and can catch header regressions.
- .gitignore: ignore root-level Playwright artifacts (test-results/, report).
- docs: USER_JOURNEYS §10 and SECURITY-BACKLOG updated to match the implemented
  read-only-ban behavior and the export-path fix.

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

frontend/src/lib/auth.ts

@@ -83,6 +83,9 @@ export function clearAuth(): void {
 	if (!browser) return;
 	localStorage.removeItem(TOKEN_KEY);
 	localStorage.removeItem(USER_ID_KEY);
+	// Clear the display name too — on a shared device the next user shouldn't see
+	// the previous guest's name pre-filled on /join.
+	localStorage.removeItem(DISPLAY_NAME_KEY);
 	// PIN is intentionally kept so the user can recover
 	isAuthenticated.set(false);
 	// Hooks fire in registration order. Keep them dependency-free of each other —