Add a "Maintaining the blacklist" section that mirrors "Maintaining
the whitelist" and explains the manual-addition escape hatch
preserved by the three-way merge. Clarify that direct removals do
not work and must go through the whitelist instead.
- Rewrite merge_blocklists.py to sync a single blacklist from upstream
and subtract the locally-maintained whitelist
- Replace whitelist contents with subtitle + webm seed
- Remove blacklist_permissive, whitelist_with_subtitles, and all
.prev files that are no longer needed
- Rewrite README to reflect the two-file model and link to wiki