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

wiki/decisions/bodytype-classifier-training.md

@@ -173,6 +173,13 @@ The CLI is still there for debugging, inside the running container:
 - **Reviewing is the bottleneck**: the Training section shows labels per class against the
   minimum and keeps Train disabled until two classes clear it.
 
+**"Training — trainer not reachable: fetch failed" (2026-09-16).** Not a network problem: the
+trainer answered `/health` but its `/readiness` crashed on the collector's DB (a volume from before
+the `kind` column) and the stdlib server dropped the socket without a reply. Since the fix the
+trainer answers **500 JSON with the error** and the collector shows that text; a genuine network
+failure still reads "fetch failed" / ECONNREFUSED. See [[vision-review-outbox]] §Incident 2026-09-16.
+
+
 ## Packaging rule (same as the vision service)
 
 Core deps are light (numpy, opencv-headless, onnxruntime): `inspect`, `evaluate`, the data and