fix(ci): skip the snapshot and apply when no migrations are pending

[nourshoreibah]

Why

The migrate job fires whenever a push touches db/migrations/** and on every manual dispatch — a dispatch has no diff, so detect-changes assumes migrate=true:

if [[ "$EVENT" == "workflow_dispatch" ]]; then
  migrate=true

Neither condition means anything is actually pending; CI can't know that until it connects to the database.

With nothing pending the apply was already a harmless no-op (database is already up to date), but the job still took an RDS snapshot and waited for it — 1-2 minutes per deploy, and junk churning through the 5-snapshot retention window. Both of the manual dispatches on 566ffa4 hit exactly this path.

What

migrate:status already runs as a read-only preflight and already prints the count, so parse it and gate on it:

• snapshot and apply are skipped when pending == 0
• unparseable output falls through as 1, so the safe path (snapshot first) is taken
• pruning only runs when a snapshot was actually created
• the baseline tripwire still runs unconditionally

Reporting

up being skipped is a success as far as the deploy is concerned, so the result artifact and the Slack step report success rather than the raw skipped — otherwise slack-deploy's finalize would mark the whole run failed, since it fails on any .meta that isn't success. The Slack step name and the job summary say "nothing pending" instead of claiming migrations were applied.

A missing migrate.log is fine: the result step falls back to status.log, and slack-deploy already wraps its details-file read in a try/catch.

Verification

The count parser, against real migrate:status output:

status output	parsed
1 applied, 0 pending	0 → skips
1 applied, 1 pending	1 → runs
3 applied, 12 pending	12 → runs
garbage / no count	1 → runs (safe default)

Resulting step conditions:

Pending migrations (read-only preflight)      if=-
Snapshot production before migrating          if=pending != '0'
Apply migrations                              if=pending != '0'
Nothing to apply                              if=pending == '0'
Prune old pre-migration snapshots (keep 5)    if=always() && snapshot.outcome == 'success'

Production was adopted separately (ledger now holds 0000_baseline_schema, 1 applied, 0 pending), so on current main this job takes the pending == 0 path — which is what this PR makes cheap.

🤖 Generated with Claude Code