feat: randomize meme button colors

[ohld]

Summary: Adds random Telegram Bot API button styles to meme action buttons in private feed and group meme reactions, including share buttons under memes. Keeps compatibility with older local python-telegram-bot by serializing style via api_kwargs when needed. Tests: ruff check on touched files; pytest tests/tgbot/test_meme_like_count_experiment.py tests/tgbot/test_sharing.py tests/tgbot/test_group_meme_reaction.py. Broader pytest tests/tgbot was attempted but local DB hostname app_db was not resolvable in this environment.

[ohld]

STAFF ENGINEER REVIEW: APPROVED — Structural review clean. /review and Codex second opinion found no actionable correctness issues. CSO skipped because the diff only touches Telegram button rendering/tests, not sensitive surfaces. Merge remains blocked until CI is green; current blocker is lint formatting in tests/tgbot/test_meme_like_count_experiment.py.

[ohld]

❌ CI red — leaving merge blocked. Current blocker: ruff format --check src/ tests/ wants tests/tgbot/test_meme_like_count_experiment.py reformatted. Next push will re-trigger me.

[ohld]

STAFF ENGINEER REVIEW: APPROVED — Clean structural review. Telegram button style serialization preserves callback/url behavior; no SQL, secret, recommendation-weight, or sensitive-surface regressions found. Codex review found no discrete correctness issue. Local targeted checks: ruff passed; pytest tests/tgbot/test_meme_like_count_experiment.py tests/tgbot/test_sharing.py tests/tgbot/test_group_meme_reaction.py passed (15 tests).

[ohld]

✅ Approved + merged.