perf: optimize slow queries for trending/feed/community APIs

[ety001]

Based on beta-hivemind-slow-query-analysis.md report:

Database Indexes (v26 -> v27):

• Add hive_posts_cache_temp_ix6b: depth+sc_trend index for trending queries
• Add hive_posts_cache_temp_ix30b: community+depth+sc_trend index for community queries
• Add idx_feed_cache_account_created_desc: optimize feed/blog ORDER BY
• Add idx_reblogs_post_account: optimize NOT EXISTS subquery

Query Optimization:

• Replace 'author NOT IN (subquery)' with NOT EXISTS in:
  • pids_by_community()
  • pids_by_category()
• NOT EXISTS performs better than NOT IN with PostgreSQL

Expected improvement:

• Trending queries: 281ms -> ~20ms (14x faster)
• Feed queries: 30ms -> ~5ms (6x faster)
• Community queries: 23ms -> ~3ms (7x faster)
• Total CPU: ~13.6% -> ~2%