fix(bot): suppress song source link previews

[scottishwidow]

Summary

• suppress Telegram link previews for rendered song source URLs while keeping URLs visible
• apply the behavior to song result/detail/create/update messages
• add handler and navigation tests for preview suppression

Verification

• UV_CACHE_DIR=.uv-cache uv run pytest tests/test_handlers.py
• UV_CACHE_DIR=.uv-cache uv run pytest tests/test_navigation.py
• UV_CACHE_DIR=.uv-cache uv run ruff check .
• UV_CACHE_DIR=.uv-cache uv run ruff format --check .
• UV_CACHE_DIR=.uv-cache uv run mypy
• git diff --check

Note: full UV_CACHE_DIR=.uv-cache uv run pytest -q hung in this sandbox after the skipped Postgres tests; isolated tests/test_repertoire_backup_service.py also timed out, unrelated to this handler change.