Release 20260415121211

Add /stats command with video poster and reaction tracking (#25)

* Add /stats command with video poster and reaction tracking

Tracks who posts videos via /dl and how many reactions (including per-emoji
👍/👎) those posts receive. Surfaces data via a new /stats command showing
top posters, most liked/disliked videos, and top reposters per group.

- Extend DuckDB schema with video_stats table (thumbs_up/down counts, is_repost flag)
- New VideoStatsHandler records each successful /dl post including repost detection
- New StatsHandler responds to /stats with a per-group leaderboard
- Telegram: MiddlewarePoller intercepts message_reaction updates and diffs
  old/new emoji arrays to update per-emoji counts accurately
- Discord: reaction add/remove event listeners with emoji routing
- Fix ConstructTextResponseHandler overwriting textResponse for Stats action
- Fix EndOfChainHandler calling CleanupOldFingerprints before DB is initialised

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Address PR #25 review feedback

- Add allowlist check in queryTopByReactionColumn to defuse SQL injection risk
- Group leaderboard/reposter stats by user_id only, picking MAX(username) so
  a renamed user is counted as one person instead of splitting into two rows
- Drop unused reaction_count column; leaderboards already use the per-emoji
  counts so the total was dead weight
- Comment the ALTER TABLE migration block to explain why it duplicates the
  CREATE TABLE columns (needed for existing DBs created before these columns)
- StatsHandler, Discord and Telegram reaction tracking now share a single
  *sql.DB instead of opening a new DuckDB connection per event/query
- Add default case to the platform switches in StatsHandler and
  VideoStatsHandler so unknown services log and bail instead of building a
  malformed group_id

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>