fix(comments): legacy id-less comment rows superseded by attributed twins are never reconciled #266

Context

#261/#262 added comment dedup: insert_comment matches incoming comments by threads_comment_id, and backfills a legacy id-less row (threads_comment_id IS NULL) only when no exact-id row exists yet.

That leaves a permanent class of orphaned rows: legacy id-less rows fetched before #262 that already have an attributed twin in the same post. Once the attributed row exists, every future fetch takes the exact-match branch and the id-less twin is never revisited — no cleanup, migration, or reconcile path exists for it.

Evidence (production, 2026-09-11)

Post 01a084a5-75e0-7383-a87c-ad00ef90d3cb (account guyonlelucon, published 2026-09-09):

Same text, byte-identical (verified). Live re-fetch on 2026-09-11 confirmed: rows stay at 2, the attributed row updates in place, the id-less row is ignored forever. Impact: the single real comment is double-counted in sentiment (2× negative) and any per-comment workflow (reply targeting, priority).

Proposed fix

One-time reconciliation at startup (alongside existing migrations) or as a migration step: delete id-less rows that are superseded — where an attributed row with the same (post_id, text) exists:

DELETE FROM comments
WHERE threads_comment_id IS NULL
  AND EXISTS (
    SELECT 1 FROM comments c2
    WHERE c2.post_id = comments.post_id
      AND c2.text = comments.text
      AND c2.threads_comment_id IS NOT NULL
  );

Constraints to preserve (mirror #261's safety rules):

• Only supersede when the attributed row's author matches or the legacy row is anonymous (author_username IS NULL), same as the existing backfill guard.
• Never delete an id-less row that has reply_status != 'new' or stored reply_text — it carries workflow state the attributed twin lacks.
• Log the number purged (tracing::info!) for observability.

Testing

• Unit test: id-less row + attributed twin → purge removes exactly the id-less row.
• Unit test: id-less row with reply_status != 'new' → NOT purged.
• Unit test: id-less row without attributed twin → NOT purged (id-less is still the only row for Meta's standard-access behavior).