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/debugsession.py

@@ -145,6 +145,8 @@ def serialize(session: DebugSession | None, screenshots: int = 0) -> dict | None
         "ended_at": session.ended_at.isoformat() if session.ended_at else None,
         "ended_reason": session.ended_reason,
         "issue_url": session.issue_url,
+        # A send that stopped half-way (app/issues/gitea.py): the files still to go.
+        "pending_files": ((session.draft or {}).get("delivery") or {}).get("pending") or [],
         "screenshots": screenshots,
         "draft": session.draft,
     }