v2.3.0 — Bounded TikTok retries and operation progress

TikTok downloads now recover from stalled connections and response bodies within a shared operation budget instead of letting one stalled source consume the entire deadline.

• Add the opt-in NDJSON operation protocol with up to three real attempts, remaining-time propagation, cancellation, and conclusive delivery-safety results.
• Preserve streaming upload commit protection: ambiguous or partially committed Telegram sends are never blindly retried.
• Bound header/body stalls and preserve identity recovery through media inspection, with sanitized phase diagnostics.
• Prevent failed optional thumbnails from consuming retry attempts or repeating successful media preparation.
• Keep existing JSON clients compatible. Document the bot's one-second terminal-response reserve inside its 15-second total limit.

Deployment order: release and verify this backend before tt-bot v6.0.18. No database migration or new production environment variable is required for this rollout.

Included: #32