postiz: add a `temporal-db` backup participation for `temporal-postgres`

[pofallon]

Context

postiz runs two Postgres services — its own application database and a separate temporal-postgres database backing its Temporal workflow engine — but its manifest's backup block only declares one hook, for its own database. The second database is currently backed up as a live file copy with no quiesce, which is only crash-consistent, not transaction-consistent.

Hola server spec 004 (try-hola/hola#426) makes the dashboard's backup-coverage view aware of this: a deployment shaped like postiz today (accepts backup@1, one participation, two recognised database services) now renders as partially covered ("1 of 2 databases quiesced") rather than the misleading fully-covered state it showed before. This is the acceptance test for that feature (spec 004 FR-019) — postiz is used deliberately as the example of the gap, precisely because it's real and already in the catalog.

What's needed here

Add a second backup participation to postiz's manifest, once the plural backup form is accepted by the catalog schema/validator (see the companion issue, blocks this one):

"accepts": ["backup@1"],
"backup": [
  { "id": "app-db",
    "preHook":  { "service": "postiz-postgres",   "command": ["sh", "-c", "pg_dump -U <user> <db> > /backups/postiz.sql"] },
    "postHook": { "service": "postiz-postgres",   "command": ["rm", "-f", "/backups/postiz.sql"] } },
  { "id": "temporal-db",
    "preHook":  { "service": "temporal-postgres", "command": ["sh", "-c", "pg_dump -U <user> <db> > /backups/temporal.sql"] },
    "postHook": { "service": "temporal-postgres", "command": ["rm", "-f", "/backups/temporal.sql"] } }
]

(Adjust service names, users, and database names to whatever postiz's actual compose declares — check the compose bundle for the real temporal-postgres service name and credentials.)

Why

Until this lands, postiz's coverage state is honestly reported as partial by the dashboard rather than falsely reported as fully quiesced — see spec 004's Assumptions: "the catalog will adopt the plural backup form and add postiz's second participation on its own cadence; nothing here depends on it landing first. Until it does, postiz reads as partial, which is the honest state."

Depends on

• The companion issue: manifest schema + validator accepting the plural backup form.

References

• Capability contracts: support multiple participations per contract per app (postiz has an uncovered database) hola#426 (plural participation)
• specs/004-contract-cardinality/spec.md (FR-019, SC-003) in try-hola/hola