Decouple core from plugins; wire widgets hook; drop dead search hook

Architectural pass from the skill review ("plugin is the product" boundary).

Core no longer imports plugin models at module load (was a hard import-time
dependency that broke core if the computers plugin was absent/disabled):
- collector.py, applications.py, reports.py: lazy + guarded imports of the
  computers plugin models. Endpoints that need install-tracking now return 503
  when the plugin is absent instead of failing at import.

Search honors runtime enable/disable:
- search.py: _require_enabled(name) raises ImportError for a disabled plugin,
  so each plugin-scoped block skips it (a disabled plugin's rows leave search).
- Replace hardcoded root/rootpassword employee-DB connection in _search_employees
  with the shared env-backed employee_connection helper.

Plugin hooks (integrating-plugin-hooks: every hook needs a consumer):
- get_dashboard_widgets: add the consumer GET /api/dashboard/widgets (5 plugins
  already implemented the hook; it had none). Skips disabled, isolates in prod.
- get_searchable_fields: REMOVED. Zero plugins implemented it and there was no
  consumer; global search is a core concern over the asset model. Contract
  reduction, __contract_version__ 0.3.0 -> 0.4.0.

Docs/contract: PLUGIN-HOOKS.md (widgets consumer note, searchable-fields removal,
0.4.0), PLUGIN-QUICKSTART.md, ADR-001 hook list. Tests: widgets endpoint
aggregate + disabled-skip; contract tests for the removed/added hooks.

151 tests pass, naming/style green, app boots all 6 plugins.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

shopdb/__init__.py

@@ -15,7 +15,10 @@ from .plugins import plugin_manager
 # 0.3.0: shopdb.api expanded to the full plugin import surface (db, cache,
 # model bases, core models, response + pagination helpers, employee_connection)
 # so plugins no longer import internal core paths. Additive, hence minor bump.
-__contract_version__ = '0.3.0'
+# 0.4.0: removed the never-implemented get_searchable_fields hook (search is a
+# core concern over the asset model) and wired the get_dashboard_widgets hook to
+# a real consumer (/api/dashboard/widgets). Pre-1.0 contract reduction.
+__contract_version__ = '0.4.0'
 
 
 def create_app(config_name: str = None) -> Flask: