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/script.py.mako

@@ -0,0 +1,23 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+revision = ${repr(up_revision)}
+down_revision = ${repr(down_revision)}
+branch_labels = ${repr(branch_labels)}
+depends_on = ${repr(depends_on)}
+
+
+def upgrade():
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade():
+    ${downgrades if downgrades else "pass"}