BACKLOG P0-3: /api/health only ever checked *presence* of DATABASE_URL/
REDIS_URL and always returned ok:true regardless of whether anything was
reachable -- ARCHITECTURE.md's own words, "an uptime monitor watching HTTP
200 would show green for a dying instance".
lib/health-checks.ts adds real dependency probes:
- db: a live `select 1` round trip (not just DATABASE_URL presence)
- engine: a real HTTP call to the FastAPI engine's own GET /health
- stripe: Stripe's documented lightweight balance.retrieve() liveness call
- queue: honestly reports "not_integrated" when REDIS_URL is set but
unused -- no Redis client is wired anywhere in this repo, so reporting
"ok" would fabricate a check that doesn't exist
/api/health stays a liveness check (always 200 while the process can
respond; the body's `ok` field is the honest signal). New GET /api/ready
fails closed with 503 when the load-bearing dependency (db) isn't both
configured and reachable -- the "no /ready route" gap.
contracts/openapi.yaml, docs/architecture/ARCHITECTURE.md, and docs/slo.md
updated to match. 17 new unit tests (dependency-injected fakes, no real
network/DB) + 5 route-level tests.
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