Add ADR-006 generic plugin collector contract

Implements the plugin collector contract from ADR-006 so any plugin can
accept idempotent inventory ingest, not just PCs.

- base.py: add apply_collector_payload hook (companion to get_collector_schema),
  raises NotImplementedError by default for plugins that declare a schema but
  do not implement the upsert.
- collector.py: generic POST /api/collector/<plugin> dispatch with per-plugin
  API key (COLLECTOR_API_KEY_<PLUGINNAME> with COLLECTOR_API_KEY fallback),
  schema-driven identity validation, idempotent upsert, ADR-006 response
  contract (status, action, assetid, identityvalue, warnings), audit log.
  JWT-protected GET /api/collector/_schemas lists registered schemas. Legacy
  /pc, /apps, /heartbeat, /bulk kept for back-compat.
- computers plugin: implements get_collector_schema (identityfield hostname)
  and apply_collector_payload (create-or-update Asset+Computer, serialnumber,
  loggedinuser, lastboottime, primary IP communication, installed apps).
- tests: 7 collector-contract tests (auth, 404, validation, create/idempotent
  update, per-plugin key precedence, JWT schema listing).

A single dynamic dispatch route is used instead of per-plugin blueprint
registration, avoiding Flask's register-blueprint-after-first-request error.

144 tests pass, naming/style check green.

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

shopdb/plugins/base.py

@@ -120,6 +120,21 @@ class BasePlugin(ABC):
         """
         return None
 
+    def apply_collector_payload(self, payload: Dict) -> Dict:
+        """Idempotently upsert an asset from a validated collector payload.
+
+        Called by the generic /api/collector/<pluginname> endpoint after the
+        payload passed schema validation. Plugins that return a schema from
+        get_collector_schema must implement this. Return a dict with at least:
+        - 'action': 'created' | 'updated' | 'noop'
+        - 'assetid': the affected asset id (or None)
+        - 'warnings': list[str]
+        """
+        raise NotImplementedError(
+            f"{self.meta.name} declares a collector schema but does not "
+            f"implement apply_collector_payload"
+        )
+
     def on_install(self, app: Flask) -> None:
         """Called when plugin is installed via CLI."""
         pass