fix(collector,trainer): migrate an existing collector DB on open; trainer handlers answer 500 JSON

The reviewer host's collector.sqlite was created by an earlier build, before the
`kind` column. CREATE TABLE IF NOT EXISTS shapes only a new database, so every query
naming the column failed: the collector's /health (container unhealthy), every
booth ingest, and the trainer's readiness — whose stdlib server printed the
traceback and dropped the socket, which the collector could only render as
"trainer not reachable: fetch failed". Nine days like that.

- CollectorDb.#migrate(): PRAGMA table_info against the list of columns added
  since the first deploy; ALTER TABLE ADD COLUMN for each missing one (all
  nullable or defaulted). Append to that list whenever a column joins the CREATE.
  Test replays the original schema: health, ingest, stats, a legacy row reads
  back with the defaults.
- Trainer Handler._guarded(): any unexpected exception → 500 JSON naming it,
  never a dropped connection; /health keeps answering. Test drives readiness
  against an old-schema DB.
- The collector's training status proxy includes the trainer's error text.

Wiki: the incident and the schema rule (vision-review-outbox), what the message
means (bodytype-classifier-training), log. Deploy: the new collector migrates on
start; nothing manual.

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

apps/collector/src/app.ts

@@ -253,7 +253,18 @@ export async function buildCollector(cfg: CollectorConfig, opts: { dbFile?: stri
     try {
       const get = async (p: string) => {
         const r = await fetch(trainer + p, { signal: AbortSignal.timeout(15_000) });
-        if (!r.ok) throw new Error(`${p} → HTTP ${r.status}`);
+        if (!r.ok) {
+          // Surface the trainer's own error text (its handlers answer 500 JSON), so the
+          // reviewer reads "no such column: kind", not just a status code.
+          const detail = await r.text().then((t) => {
+            try {
+              return String((JSON.parse(t) as { error?: unknown }).error ?? t);
+            } catch {
+              return t;
+            }
+          }, () => "");
+          throw new Error(`${p} → HTTP ${r.status}${detail ? `: ${detail.slice(0, 300)}` : ""}`);
+        }
         return r.json() as Promise<Record<string, unknown>>;
       };
       const [health, readiness, versions, jobs] = await Promise.all([get("/health"), get("/readiness"), get("/versions"), get("/jobs")]);