feat(trainer): training from the collector UI — the trainer becomes a job service, the review page gains a Training section

Trainer: `parking-trainer serve` — a stdlib HTTP job API on the compose network (never
published): /health, /readiness, /versions, /versions/<v>/report, /jobs. One job at a
time; each job runs the CLI as a subprocess with its output captured, state + log
persisted under /out/jobs/ so a restart keeps history. `publish` takes its URL from
TRAINER_PUBLISH_URL. Dockerfile: CMD serve, EXPOSE 8091, healthcheck.

Collector: COLLECTOR_TRAINER_URL + /api/training/{status,jobs,jobs/:id,versions/:v/report}
— a reviewer-gated proxy that forwards a fixed set of paths and whitelisted knobs and
passes the trainer's status codes through (409 while a job runs; 503 unconfigured, 502
unreachable). /review gains the Training section: labels per class vs the minimum with
Train disabled until two classes clear it, mode / backbone / floor, the running job's
live log, the versions with Report / Evaluate / Publish (publish confirms), and the
reminder that pinning stays a git commit. Fixed on the way: an apostrophe in the page's
inline script broke the whole page — a test now parses the script.

Compose: `trainer` is a service (restart: unless-stopped, read-only data volume, its own
trainer-out volume), the `train` profile and TRAINER_OUT are gone; the Docker-socket
route was rejected (root on the host for a service booths upload to). Verified with both
images running together: a Train started through the proxy finished, version and report
came back, the page rendered.

Wiki: bodytype-classifier-training (loop, running it, operating notes superseded),
vision-review-outbox, fleet-deployment-komodo, log.

Claude-Session: https://claude.ai/code/session_01FWncR69HgGPuei1dLrW3cU

apps/collector/src/config.ts

@@ -6,6 +6,9 @@ export interface CollectorConfig {
   readonly boothTokens: ReadonlyMap<string, string>;
   /** The single reviewer login; null = review screen and export refuse (503). */
   readonly reviewer: { readonly user: string; readonly pass: string } | null;
+  /** The trainer's job API on the compose network (http://trainer:8091); null = the
+   *  Training section is hidden and /api/training/* answers 503. */
+  readonly trainerUrl: string | null;
 }
 
 /** "booth-7:abc,booth-9:def" (commas, whitespace or newlines between pairs). */
@@ -32,5 +35,6 @@ export function configFromEnv(env: NodeJS.ProcessEnv = process.env): CollectorCo
     dataDir: env.COLLECTOR_DATA_DIR ?? "/data",
     boothTokens: parseBoothTokens(env.COLLECTOR_BOOTH_TOKENS ?? ""),
     reviewer: user && pass.length >= 8 ? { user, pass } : null,
+    trainerUrl: (env.COLLECTOR_TRAINER_URL ?? "").trim().replace(/\/+$/, "") || null,
   };
 }