feat(channels): record and show whether a channel proposal was auto- or admin-approved #616

Problem

With channelProposals.autoApprove on, an admin cannot see which approved channels were auto-approved and which were approved by hand. channel_proposals stores only status, created_at and reviewed_at (cmd/ingestor/channel_proposals.go, submitChannelProposal). Today the only way to tell is to read the database and infer it from reviewed_at ≈ created_at.

Proposal

1. Schema (ingestor; the server stays read-only). Add a nullable column channel_proposals.approved_by TEXT with the values 'auto' or 'admin'.
   • Add it as an idempotent migration: ALTER TABLE … ADD COLUMN when missing.
   • Register it with the self-healing optional-column detection on the server side, so a server/ingestor boot race cannot hide it.
2. Writes (ingestor).
   • submitChannelProposal sets 'auto' when autoApprove approves a new name.
   • Admin approve sets 'admin'.
   • Reject and revoke clear it. A resubmitted (revoked → pending) row also clears it.
3. Backfill (one-off, ingestor migration). For existing approved rows, set 'auto' where reviewed_at - created_at < 60000 and otherwise 'admin'. Document that values from before the migration are inferred.
4. API / UI.
   • Expose approvedBy on the proposals list used by #/channels?view=proposals, as a named struct field (no map[string]interface{}).
   • Show it as a small badge ("auto" / "admin") and add a filter "Auto-approved" to that view.
   • Reflect the filter in the URL hash (deep link), use CSS variables only, and make it work at 390px.

Tests

• Ingestor:
  • auto-approve writes 'auto', admin approve writes 'admin', reject and revoke clear it, and resubmit resets it;
  • the migration is idempotent;
  • the backfill classifies the fixture rows correctly.
• Server:
  • the field is present when the column exists and omitted or null on an old DB without it;
  • the read-only invariant holds.
• Frontend:
  • unit test for the filter and badge;
  • Playwright test for the filter and its deep link.

Relates to the shared hashtag channels feature (channelProposals).