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>

.gitignore

@@ -22,6 +22,9 @@ e2e/playwright-report/
 e2e/test-results/
 e2e/.cache/
 e2e/.env.test
+# Playwright artifacts when run from the repo root instead of e2e/
+/test-results/
+/playwright-report/
 
 # OS
 .DS_Store