Separate cache clearing from migrate, add clear_rxdjango_cache command

[lfagundes]

• Split MongoSignalWriter.init_database() into ensure_indexes() (idempotent, always runs on post_migrate) and clear_cache() (delete_many instead of drop, preserving indexes). init_database() kept for backward compat with tests.
• Add RX_CLEAR_CACHE_ON_MIGRATE setting (default True). When True, cache is cleared only when the migration plan includes migrations for the sender app, skipping no-op migrate runs. Set False for zero-downtime blue/green deploys.
• Add clear_rxdjango_cache management command for explicit cache lifecycle control, with --channel and --dry-run options.
• Update docs/caching.rst with new setting, command reference, and zero-downtime deployment workflows.