Document migration-on-deploy contract for plugin schema changes

A plugin model change that references a column its migration has not applied yet 500s every query on that table. Make the deploy order explicit: run flask db upgrade then flask plugin upgrade-all before serving traffic, and require model+migration to ship together.

.github/copilot-instructions.md

@@ -67,6 +67,11 @@ name. Spell out: `config context manager request response environment utilities`
   `migrations/versions/`; each plugin owns its own chain under
   `plugins/<name>/migrations/`. New schema = a new migration with an idempotent
   guard and a real downgrade. Migrations must run clean on strict MySQL 8.
+  Any model column/table change ships WITH its migration in the SAME change, and
+  bringing up a new image MUST run `flask db upgrade` then
+  `flask plugin upgrade-all` before serving traffic. A model that references a
+  column its migration has not applied yet 500s every query on that table
+  (`1054 Unknown column`), so code and schema must never be deployed out of step.
 - **Asset model is the platform contract.** Physical things are an `Asset` plus a
   plugin subtype row linked by `assetid` (FK, `ON DELETE CASCADE`). Consumables
   with quantities are standalone tables, not assets.

CONTRIBUTING.md

@@ -24,6 +24,24 @@
 - **Index names**: `idx_<table>_<column>`. Underscores allowed here only because indexes are infrastructure, not data.
   - Good: `idx_machines_locationid`
 
+### Migrations and deploy ordering
+
+- **A model change ships WITH its migration in the same change.** Adding or
+  changing a column/table on a SQLAlchemy model without the matching Alembic
+  migration is a broken change, even if it passes tests against a freshly built
+  schema.
+- **Code and schema must never deploy out of step.** A model that references a
+  column its migration has not applied yet makes MySQL raise
+  `1054 Unknown column`, which 500s every query on that table (this is not a
+  degraded read - the whole endpoint fails).
+- **Bringing up a new image MUST run migrations before serving traffic**, in
+  order: `flask db upgrade` (core chain) then `flask plugin upgrade-all`
+  (per-plugin chains). A rebuilt image with new model code but an
+  un-migrated database will 500 until this runs.
+- Core schema lives in `migrations/versions/`; each plugin owns its own chain
+  under `plugins/<name>/migrations/`. New schema = a new migration with an
+  idempotent guard and a real downgrade, clean on strict MySQL 8.
+
 ### Allowed acronyms (closed list, expand by PR)
 
 - **Universal**: `id`, `url`, `api`, `http`, `https`, `json`, `jwt`, `sql`, `os`, `ip`, `dns`, `csv`, `pdf`, `cors`, `ttl`, `uuid`, `html`, `css`, `orm`