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/core/api/applications.py

@@ -7,13 +7,49 @@ from shopdb.extensions import db
 from shopdb.core.models import (
     Application, AppVersion, AppOwner, SupportTeam, AuditLog
 )
-from plugins.computers.models import Computer, ComputerInstalledApp
 from shopdb.utils.responses import (
     success_response,
     error_response,
     paginated_response,
     ErrorCodes
 )
+
+
+def _computer_models():
+    """Lazily import the computers plugin models, or None if unavailable.
+
+    Application install-tracking is a join over the computers plugin's tables.
+    Importing lazily keeps the applications API importable when the computers
+    plugin is absent or disabled.
+    """
+    try:
+        from plugins.computers.models import Computer, ComputerInstalledApp
+        return Computer, ComputerInstalledApp
+    except ImportError:
+        return None
+
+
+def _installed_count(appid):
+    """Count active installs of an app, 0 when the computers plugin is absent."""
+    models = _computer_models()
+    if not models:
+        return 0
+    _, ComputerInstalledApp = models
+    return ComputerInstalledApp.query.filter_by(appid=appid, isactive=True).count()
+
+
+def _require_computer_models():
+    """Resolve (Computer, ComputerInstalledApp) or a 503 response tuple.
+
+    Usage: `models, err = _require_computer_models(); if err: return err`.
+    """
+    models = _computer_models()
+    if not models:
+        return None, error_response(
+            ErrorCodes.INTERNAL_ERROR,
+            'Install tracking requires the computers plugin',
+            http_code=503)
+    return models, None
 from shopdb.utils.pagination import get_pagination_params, paginate_query
 
 applications_bp = Blueprint('applications', __name__)
@@ -66,8 +102,7 @@ def list_applications():
             }
         else:
             app_dict['supportteam'] = None
-        app_dict['installedcount'] = ComputerInstalledApp.query.filter_by(
-            appid=app.appid, isactive=True).count()
+        app_dict['installedcount'] = _installed_count(app.appid)
         data.append(app_dict)
 
     return paginated_response(data, page, per_page, total)
@@ -97,8 +132,7 @@ def get_application(app_id: int):
     else:
         data['supportteam'] = None
     data['versions'] = [v.to_dict() for v in app.versions.filter_by(isactive=True).order_by(AppVersion.version.desc()).all()]
-    data['installedcount'] = ComputerInstalledApp.query.filter_by(
-        appid=app.appid, isactive=True).count()
+    data['installedcount'] = _installed_count(app.appid)
 
     return success_response(data)
 
@@ -263,6 +297,11 @@ def list_installed_machines(app_id: int):
     if not app:
         return error_response(ErrorCodes.NOT_FOUND, 'Application not found', http_code=404)
 
+    models, err = _require_computer_models()
+    if err:
+        return err
+    Computer, ComputerInstalledApp = models
+
     installed = ComputerInstalledApp.query.filter_by(
         appid=app_id, isactive=True).all()
     data = []
@@ -293,6 +332,11 @@ def list_installed_machines(app_id: int):
 @jwt_required(optional=True)
 def list_machine_applications(machine_id: int):
     """List all applications installed on a computer."""
+    models, err = _require_computer_models()
+    if err:
+        return err
+    Computer, ComputerInstalledApp = models
+
     comp = Computer.query.get(machine_id)
     if not comp:
         return error_response(ErrorCodes.NOT_FOUND, 'Computer not found', http_code=404)
@@ -305,6 +349,11 @@ def list_machine_applications(machine_id: int):
 @jwt_required()
 def install_application(machine_id: int):
     """Install an application on a computer."""
+    models, err = _require_computer_models()
+    if err:
+        return err
+    Computer, ComputerInstalledApp = models
+
     comp = Computer.query.get(machine_id)
     if not comp:
         return error_response(ErrorCodes.NOT_FOUND, 'Computer not found', http_code=404)
@@ -351,6 +400,11 @@ def install_application(machine_id: int):
 @jwt_required()
 def uninstall_application(machine_id: int, app_id: int):
     """Uninstall an application from a computer."""
+    models, err = _require_computer_models()
+    if err:
+        return err
+    Computer, ComputerInstalledApp = models
+
     installed = ComputerInstalledApp.query.filter_by(
         computerid=machine_id,
         appid=app_id,
@@ -370,6 +424,11 @@ def uninstall_application(machine_id: int, app_id: int):
 @jwt_required()
 def update_installed_app(machine_id: int, app_id: int):
     """Update installed application (e.g., change version)."""
+    models, err = _require_computer_models()
+    if err:
+        return err
+    Computer, ComputerInstalledApp = models
+
     installed = ComputerInstalledApp.query.filter_by(
         computerid=machine_id,
         appid=app_id,