fix(frontend-admin): SSE/AbortController hygiene + signal-chain through proxy (0.87.9)

Three frontend findings (medium):

1. **`hooks.server.ts` didn't chain client-disconnect into the upstream
   fetch.** Closing an admin tab left the backend's `broadcast::Receiver`
   + spawned tokio task running until next backend restart. Listen for
   `event.request.signal.abort` and forward to the timeout controller.

2. **`admin/analysis/+page.svelte` missing visibility/pagehide/pageshow
   handlers.** Mobile Safari throttles SSE in background tabs; bfcache
   leaves a dead connection on back-nav. Apply the close/open-stream
   pattern from `admin/crawler/+page.svelte`.

3. **`loadCoverage` had no AbortController.** Debounced search
   keystrokes raced. Per-loader controller plumbed through
   `getAnalysisMangaCoverage`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/Cargo.lock

@@ -1517,7 +1517,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.87.8"
+version = "0.87.9"
 dependencies = [
  "anyhow",
  "argon2",