Phase 2: define plugin contract surface and add compliance tests

Locks the public surface plugin authors at sister sites depend on.

Contract version (shopdb/__init__.py):
- __contract_version__ = '0.1.0'. Per ADR-002, plugins declare a
  compatible range in manifest.json `core_version`. Pre-1.0 signals
  the contract is still settling; sister sites should pin tightly.

BasePlugin hook changes (shopdb/plugins/base.py):
- Add get_collector_schema() per ADR-006. Returns JSON Schema (with
  identityfield + fields) describing the payload of an external
  collector pushing to /api/collector/<pluginname>. Defaults to None
  (no auto-registered endpoint).
- Remove get_event_handlers(). Event bus deferred indefinitely per
  ADR-001 (no real use case yet; add via new ADR if it appears).

Hook reference (docs/PLUGIN-HOOKS.md):
- Canonical reference for the contract: required hooks (meta,
  get_blueprint, get_models), optional hooks (init_app,
  get_cli_commands, get_services, get_dashboard_widgets,
  get_navigation_items, get_searchable_fields, get_collector_schema),
  lifecycle hooks (on_install, on_uninstall, on_enable, on_disable),
  helpers exposed in shopdb.api (audit_log, Setting,
  resolve_asset_position).
- Versioning rules + change-classification guidance.

Compliance tests (tests/test_plugin_contract.py):
- 8 distinct contract assertions parametrized over 6 bundled plugins
  (computers, equipment, network, notifications, printers, usb).
- Asserts: subclasses BasePlugin; manifest has required fields; meta
  returns valid PluginMeta; get_blueprint returns Blueprint or None;
  get_models returns model classes; get_collector_schema returns
  None or {identityfield, fields}; get_navigation_items and
  get_searchable_fields return list.
- Plus 3 framework-level: __contract_version__ is valid semver,
  get_event_handlers absent, get_collector_schema present.

Test count: 15 -> 66 passing.

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

shopdb/plugins/base.py

@@ -73,9 +73,21 @@ class BasePlugin(ABC):
         """Return dict of service name -> service class."""
         return {}
 
-    def get_event_handlers(self) -> Dict[str, callable]:
-        """Return dict of event name -> handler function."""
-        return {}
+    def get_collector_schema(self) -> Optional[Dict]:
+        """Return JSON Schema describing the collector payload for this plugin.
+
+        Return None if the plugin does not accept collector input.
+
+        See ADR-006 for the contract. The schema must include:
+        - 'identityfield': name of the field that uniquely identifies an
+          asset across submissions (e.g., 'hostname' for PCs,
+          'macaddress' for network devices). Used for idempotent upsert.
+        - 'fields': JSON Schema definitions for the rest of the payload.
+
+        Plugins returning a non-None schema have an endpoint at
+        /api/collector/<pluginname> auto-registered by the loader.
+        """
+        return None
 
     def on_install(self, app: Flask) -> None:
         """Called when plugin is installed via CLI."""