feat(platform-db): add lab host and environment registry

New Phase 1 Platform database — PostgreSQL, separate from the Lab image's own
SQLite (which keeps its current students/progress data untouched, decision #2 /
correction C in docs/LAB_ARCHITECTURE.md).

Schema (platform/provisioner/migrations/):
- lab_hosts: the abstraction that keeps Docker connectivity out of code —
  endpoint (unix:// or tcp://+TLS), enabled flag. Phase 1 ships one row.
- environments: environment_id (UUID), student_id (opaque external identifier,
  no FK — students table isn't in this database yet), lab_host_id (real FK),
  status, routing_token (CSPRNG via crypto.randomBytes, opaque), container/
  network identity, resource limits, lifecycle timestamps.
- environment_events: append-only audit trail, one row per lifecycle
  transition, never logs the routing token in plaintext (see
  lifecycle.js's redact()).

File-based migration runner (db.js) matching the check-before-alter idempotency
pattern already used in the Lab image's backend/server.js, applied on every
Provisioner boot.

Resource limits and the lab_hosts seed are config files
(config/resource-policy.yaml, config/lab-hosts.seed.yaml), not database tables —
there's no admin UI to manage them via a database yet, so a DB-backed policy
table would just be a write path nothing uses.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

platform/provisioner/config/resource-policy.yaml

@@ -0,0 +1,22 @@
+# Phase 1 resource policy — config-driven, not hard-coded (decision #4).
+# No specific production numbers chosen yet; target host class for planning is ~16 cores/32
+# threads, 128GB RAM, SSD/NVMe. Actual per-lab limits get set by benchmarking (see
+# docs/PHASE1_PLATFORM_FOUNDATION.md §8 test 9), not guessed backward from the spec sheet.
+# These defaults are sized for local single-machine Phase 1 dev/testing only.
+
+default:
+  cpu_limit: 2.0          # vCPUs
+  mem_limit_mb: 4096
+  pids_limit: 512
+  storage_limit_mb: 8192  # only enforced if the host's Docker storage driver supports
+                           # --storage-opt size= (see known-limitations in the SOP)
+
+idle_threshold_minutes: 30       # past this: log a warning event only
+abandoned_threshold_minutes: 120 # past this: reaper destroys for real
+
+reaper_poll_interval_seconds: 60
+
+health_check:
+  path: /api/health
+  timeout_ms: 240000   # k3s cold start in a nested container can be slow — matches the
+                        # ~60-attempt wait already in scripts/entrypoint.sh