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>

backend/src/state.rs

@@ -36,8 +36,12 @@ pub struct AppState {
 impl AppState {
     pub fn new(pool: PgPool, config: AppConfig) -> Self {
         let (sse_tx, _) = broadcast::channel(256);
-        let compression =
-            CompressionWorker::new(pool.clone(), config.media_path.clone(), 2, sse_tx.clone());
+        let compression = CompressionWorker::new(
+            pool.clone(),
+            config.media_path.clone(),
+            config.compression_concurrency,
+            sse_tx.clone(),
+        );
         Self {
             pool,
             config,