Make keyword optional and resumable for Keyword Search + Forward

[klept0]

Summary

• keyword is no longer required for Keyword Search + Forward (CLI dialog and POST /api/keyword-forward) — leaving it blank now forwards every message in the selected range instead of blocking with a "Keyword cannot be empty" validation error.
• When a date range is selected, keyword forwarding is now resumable: re-running the exact same source/date-range/keyword combination checks what was already forwarded and only sends what hasn't been, whether the previous run finished or was interrupted.
• Dry-run now reports how many messages remain to forward, instead of always returning 0.

Implementation

Extracted the progress-tracking logic already used by Past Forward Messages/Forward Media Files (Forward.py) into a small, dependency-free source/service/ForwardProgress.py module (just JSON read/write — no Telethon client/queue needed), so MessageService.forward_messages_by_keyword can reuse the exact same resume/dedup mechanism instead of a third, divergent implementation. Forward.py's own progress_key/_load_progress/_save_progress/_mark_progress_completed/clear_progress are now thin delegating wrappers, so its existing public API (and the tests exercising it) are unchanged.

Test plan

• ruff check / ruff format --check — clean
• mypy source/ web/app.py — clean
• pytest tests/ — 23/23 pass, including new regression tests: keyword-optional forwarding, resume-skips-already-forwarded-messages, and dry-run reporting the real remaining count
• Verified live via a preview server: the web form's keyword field no longer has required, and POST /api/keyword-forward accepts keyword: null without a validation error (reaches business logic, confirmed via a 503 "not initialized" response rather than a 422)