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>

e2e/Caddyfile.test

@@ -4,7 +4,17 @@
 # of HTTPS/Let's Encrypt.
 
 :3101 {
-    encode zstd gzip
+    # Mirror prod: exclude the SSE stream from compression so buffering doesn't
+    # delay real-time events (and so the test stack exercises the real behavior).
+    @compressible not path /api/v1/stream
+    encode @compressible zstd gzip
+
+    # Mirror prod's security headers (minus HSTS, which is HTTPS-only).
+    header {
+        X-Content-Type-Options "nosniff"
+        X-Frame-Options "DENY"
+        Referrer-Policy "strict-origin-when-cross-origin"
+    }
 
     reverse_proxy /api/* app:3000
     reverse_proxy /media/* app:3000