harden: stream-token host allowlist (SSRF), CSRF header on mutating routes, atomic JSON writes

- /stream/{token}: tokens are plain base64; allowlist https://hls.anidb.app
  so a session can't use the proxy for arbitrary URLs (new test)
- X-Aniweb:1 required on POST/DELETE except /login — SameSite=Lax is
  still sent on cross-site form POSTs, which could hit /account/delete
  (new test; SPA + tests send the header)
- tmp+os.replace writes for users/history/searches: a crash mid-write
  could corrupt the JSON -> load() returned {} -> silent data loss
- resume positions now read from full history (?all=1), not top-5
- pagehide bound once; savePos reads currentEp (no stale closures)
- account delete also removes the searches file; episodes TTL 5m
- ani-web.service: supervised restart (not installed; see header)
- gitignore pi-session dumps + .pytest_cache

.gitignore

@@ -4,3 +4,5 @@ __pycache__/
 *.tar.gz
 server/data/
 server.log
+pi-session-*.html
+.pytest_cache/