ADR-014 Phase 2: flask plugin prune-schema for lean per-site DBs

A lean site still gets every plugin's tables from the shared core Alembic
baseline. prune-schema drops the tables of plugins not installed on this
site, leaving core + chosen-plugin tables, with no edit to any released
migration (the relocate-into-plugin-baselines alternative would mean
rewriting ~15 released core migrations for a cosmetic gain - see ADR-014).

- shopdb/plugins/cli.py: prune-schema command. Dry-run by default; --yes to
  execute; refuses non-empty tables without --force. Drops by table name (no
  plugin import) so it works on a lean image. MySQL: private AUTOCOMMIT engine
  (db.engine's pooled connections sit idle-in-transaction in a CLI context and
  would deadlock the DROP on a metadata lock). SQLite: db.engine, restoring the
  prior foreign_keys pragma so the StaticPool connection is not left changed.
- tests/test_plugin_prune_schema.py: drop-only-not-installed, full no-op,
  refuse-non-empty, force-drops-non-empty.
- docs/DEPLOY.md: lean provisioning step after upgrade-all.
- ADR-014 ACCEPTED; index updated.

Verified on MySQL: full install then prune = no-op (86 tables); lean install
(machines+printers) then prune drops the other 19 plugin tables; second run
no-op. Full suite 1077 passed.

docs/DEPLOY.md

@@ -73,6 +73,24 @@ any plugin-specific migrations added after the ownership cutover. Both commands
 are idempotent, so re-running them is safe. See ADR-008 for why plugin schema
 splits into per-plugin chains from the cutover forward.
 
+**Lean sites (ADR-014):** the core chain creates every bundled plugin's tables,
+so a site that ships only some plugins still has the others' (empty) tables. To
+carry only core + chosen-plugin tables, prune the rest once, at initial
+provisioning, after the two commands above:
+
+```bash
+docker compose exec api flask plugin prune-schema            # dry-run, review
+docker compose exec api flask plugin prune-schema --yes --force
+```
+
+It drops the tables of every plugin not installed on this site. `--force` is
+needed because the core chain seeds a few plugin reference tables (default
+access protocols, etc.); at first provisioning those hold only seeded defaults,
+before any site data. It refuses to drop a table that holds rows without
+`--force`, so it is safe to leave out of routine upgrades - run it only when
+provisioning a lean site or after deliberately removing a plugin. Installing a
+pruned plugin later recreates its tables automatically.
+
 **Charset:** the schema is utf8mb4 (`utf8mb4_unicode_ci`). The docker-compose `db` service sets `--character-set-server=utf8mb4`, so the auto-created `shopdb_flask` database is utf8mb4. If you point at an external MySQL instead of the bundled container, create the database as utf8mb4 first, or it inherits the server default (often latin1) and the schema silently drifts:
 
 ```sql