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/config.py

@@ -351,6 +351,13 @@ class Settings(BaseSettings):
     tmdb_api_base: str = "https://api.themoviedb.org/3"
     # Metadata language for TMDB fetches (keyword/overview locale). Stored EN by default.
     tmdb_language: str = "en-US"
+
+    # --- Bug reports (E16): where a debug session's issue goes. All three empty = no tracker; the
+    # composer still exports a zip. The repository must be PRIVATE — an issue carries the reporter's
+    # log — and app/issues/gitea.py refuses a public one. Runtime knobs (sysconfig, admin UI). ---
+    issues_gitea_url: str = ""
+    issues_gitea_repo: str = ""
+    issues_gitea_token: str = ""
     # How often the TMDB enrichment job fetches a batch of not-yet-enriched Plex titles, in minutes.
     tmdb_enrich_interval_min: int = 120
     # Titles fetched per TMDB enrichment run (TMDB is fast + generously rate-limited).