Phase 7B: per-plugin Alembic chains for bundled plugins

Each of the six bundled plugins (computers, equipment, network,
notifications, printers, usb) now has its own Alembic chain with a
baseline migration. Sister sites adopting one of these plugins can
manage its schema via `flask plugin migrate <name>` instead of relying
on db.create_all to bootstrap everything.

Existing single-site deploys that bootstrap via db.create_all continue
to work unchanged. The chains coexist; the bootstrap path stays the
operator's choice.

Framework
- shopdb/plugins/alembic_template.py: shared env.py logic + helpers.
  PLUGIN_TABLE_OWNERS pins which tables belong to which plugin (explicit
  registry, not import-side-effect). _get_plugin_metadata filters
  db.metadata to only the named plugin's tables. create_plugin_tables /
  drop_plugin_tables emit DDL via SQLAlchemy CreateTable so the table
  definitions stay sourced from the models, not duplicated.
- shopdb/plugins/__init__.py: PluginManager.upgrade_all_plugins() runs
  pending migrations across every discovered plugin and returns a status
  dict. Idempotent (Alembic skips applied revisions).

CLI
- `flask plugin upgrade-all` runs pending migrations for every plugin.
  Used on a fresh deploy after the core schema is in place.

Per-plugin scaffolding
- plugins/{computers,equipment,network,notifications,printers,usb}/
  migrations/{alembic.ini, env.py, script.py.mako, versions/0001_baseline.py}
- Each env.py is a 5-line shim that sets PLUGIN_NAME and delegates to
  the shared template. Each 0001_baseline calls create_plugin_tables(name)
  / drop_plugin_tables(name); no duplication of column definitions.

Tests
- tests/test_plugin_migrations.py (18 cases): every bundled plugin has
  an entry in PLUGIN_TABLE_OWNERS, has the on-disk Alembic scaffolding,
  and the filtered MetaData contains every owned table (catches drift
  between the template's table list and what the models declare).
- 129 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugins/equipment/migrations/alembic.ini

@@ -0,0 +1,30 @@
+[alembic]
+script_location = .
+prepend_sys_path = .
+file_template = %%(rev)s_%%(slug)s
+
+[logging]
+keys = root
+
+[loggers]
+keys = root
+
+[handlers]
+keys = console
+
+[formatters]
+keys = generic
+
+[logger_root]
+level = INFO
+handlers = console
+qualname =
+
+[handler_console]
+class = StreamHandler
+args = (sys.stderr,)
+level = INFO
+formatter = generic
+
+[formatter_generic]
+format = %%(levelname)-5.5s [%%(name)s] %%(message)s