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/adr/README.md

@@ -26,7 +26,7 @@ Each ADR captures a single architectural decision: the context, the decision its
 | [011](ADR-011-machines-rename.md) | Machines rename + modeltypes retyping | ACCEPTED |
 | [012](ADR-012-geenforce-manifest-ownership.md) | GE-Enforce manifest ownership in shopdb | ACCEPTED |
 | [013](ADR-013-plugin-catalog-and-lean-builds.md) | Plugin catalog, curated shelf, and lean per-site builds | PROPOSED |
-| [014](ADR-014-schema-lean-per-site.md) | Schema-lean per-site builds (retire cross-plugin FKs, lift plugin tables) | PROPOSED |
+| [014](ADR-014-schema-lean-per-site.md) | Schema-lean per-site builds (retire cross-plugin FKs, prune not-installed plugin tables) | ACCEPTED |
 
 ## Authoring