#123 · Rollup retention + outline_url NULL semantics

Context

Two related cleanup items from the onboarding feature's team-review:

A. Outline doc accumulation, no retention (security WARNING) The 02:00 ET cron creates a new Outline document every day, indefinitely. After 12 months that's ~365 docs in the Community Digest collection, all titled "Community Digest — week of …" with overlapping 7-day windows. Collection becomes unsearchable; older docs drift past mental "this week" and become a discoverability surface for old aggregated activity.

B. outline_url NOT NULL semantics conflict (architecture WARNING) apps/signal-bot/database/060_community_rollups.sql declares outline_url TEXT NOT NULL, but community-rollup.ts writes outline_url: '' for no_activity and failed paths because no doc was actually created. The getLatestCommunityRollup query filters outline_url <> '' to compensate, but semantically it should be NULL.

Acceptance criteria

For (A) — choose one of two strategies and implement:

Option 1: Update-in-place (preferred) — keep one rolling "Community Digest" doc that's overwritten daily. URL stable, search bookmarked.

• [ ] Track community_digest_doc_id in a single-row config table OR reuse daily_community_rollups with a is_canonical flag
• [ ] On 02:00 cron: if a canonical doc exists, outlineClient.updateDocument(docId, newMarkdown); else create + flag canonical
• [ ] Welcome flow continues to use getLatestCommunityRollup (now always returns the same canonical URL)

Option 2: Per-day docs with monthly archival

• [ ] Keep current per-day creation
• [ ] Monthly cron (e.g., 03:00 ET on day 1): archive (Outline documents.archive) all rollup docs older than 30 days
• [ ] daily_community_rollups.archived_at column to track
• [ ] Welcome flow unchanged

For (B):

• [ ] New migration apps/signal-bot/database/061_outline_url_nullable.sql:

  ALTER TABLE daily_community_rollups ALTER COLUMN outline_url DROP NOT NULL;
  -- Backfill: any historical rows with empty string become NULL
  UPDATE daily_community_rollups SET outline_url = NULL WHERE outline_url = '';

• [ ] Update getLatestCommunityRollup filter from outline_url <> '' to outline_url IS NOT NULL
• [ ] Update persistRow to write null (not '') for non-success rows

Technical hints

• Outline's documents.update API: https://www.getoutline.com/developers#tag/documents/POST/documents.update
• Option 1 (update-in-place) is cheaper, avoids the doc-glut problem entirely, and keeps the welcome URL stable forever — recommended unless there's a specific reason to keep weekly snapshots
• If you go Option 2, also add an admin command to manually archive an old rollup doc

--- *M-tier (100 pts + 40 merge_pr = 140). Combined cleanup; covers schema fix + retention. Half-day of work.*