feat(diagnostics): send a bug report to a private Gitea issue tracker (E16 S4)

- admin config group "Bug reports": Gitea address, repository, token (secret),
  and "Check the connection" — the repository must answer, take issues and
  be PRIVATE (a report carries its reporter's log); checked on every send too
- POST /api/debug-sessions/{id}/issue sends the SAVED draft: create the issue,
  upload each file on its own request (the measured API), then show the
  screenshots inline; progress is kept in the draft, so a send that stops
  half-way keeps its issue and the retry sends only what is missing; 5 new
  issues an hour per user; audited as issue_create
- the composer: Create issue (only where a tracker is set up, naming the
  repository), a partial-send banner with retry, and a Preview — the tracker's
  own rendering in a sandboxed frame, or the zip's markdown without one
- the recorder's refusals answer in the localized family (debug_session_*,
  screenshot_*, issue(s)_*), not bare codes the UI could only call generic
- a tracker refusal logs the status only: its message can quote the title back

backend/app/sysconfig.py

@@ -192,6 +192,10 @@ SPECS: tuple[ConfigSpec, ...] = (
     # --- TMDB enrichment (E5 S2; optional) — the API key is the only secret ---
     ConfigSpec("tmdb_api_key", "str", "tmdb", "tmdb_api_key", secret=True),
     ConfigSpec("tmdb_language", "str", "tmdb", "tmdb_language"),
+    # --- Bug reports (E16): the Gitea issue tracker a debug session's issue goes to ---
+    ConfigSpec("issues_gitea_url", "str", "issues", "issues_gitea_url"),
+    ConfigSpec("issues_gitea_repo", "str", "issues", "issues_gitea_repo"),
+    ConfigSpec("issues_gitea_token", "str", "issues", "issues_gitea_token", secret=True),
     # (tmdb_enrich_interval_min is a scheduler job, tuned from the Scheduler dashboard; tmdb_enrich_batch
     # is a config.py default — both stay out of SPECS, like plex_sync_interval_min.)
 )