feat(plugins): secure installer — manifest, discovery, safe extract/fetch/scan (M4)

Plugins placed on the /plugins volume are now discovered, validated and registered
as inactive, ready to activate.

- manifest.ts: strict trek-plugin.json validation (id/version/type, known
  permissions only, egress required with http:outbound, native modules rejected).
- discovery.ts: scans the volume on startup + on demand (POST /api/admin/plugins/
  rescan), upserts rows INACTIVE, refreshes settings-field descriptors, and never
  downgrades or wipes an already-installed plugin's status / grants / config.
  Invalid or native-carrying plugins are skipped and logged.
- Activation now grants the DECLARED permissions (the consent gate) and persists
  them before spawning.
- install/ utilities for the registry installer (M5), each independently tested:
  - safe-fetch: host allowlist (GitHub only) + private-IP refusal + manual
    redirect following + size cap + sha256 (constant-time compare).
  - safe-extract: zip/tar-slip-safe extraction with its own minimal tar.gz + zip
    readers; rejects traversal, absolute paths, symlinks, oversized/too-many
    entries, and unsupported formats.
  - native-scan: refuses .node / binding.gyp / prebuilds, never follows symlinks.

server/tests/integration/plugins/plugin-runtime.test.ts

@@ -51,7 +51,7 @@ beforeAll(() => {
     };`,
   );
 
-  testDb.prepare("INSERT INTO plugins (id, status, granted_permissions, config) VALUES ('counter','active','[\"db:own\"]','{}')").run();
+  testDb.prepare("INSERT INTO plugins (id, status, permissions, config) VALUES ('counter','active','[\"db:own\"]','{}')").run();
   runtime = new PluginRuntimeService();
 });
 
@@ -124,7 +124,7 @@ describe('PluginRuntimeService (M2 end-to-end)', () => {
   it('tolerates malformed granted_permissions / config JSON on activate', async () => {
     fs.mkdirSync(path.join(codeRoot, 'messy', 'server'), { recursive: true });
     fs.writeFileSync(path.join(codeRoot, 'messy', 'server', 'index.js'), 'module.exports = { async onLoad() {} };');
-    testDb.prepare("INSERT INTO plugins (id, status, granted_permissions, config) VALUES ('messy','inactive','not-json','not-json')").run();
+    testDb.prepare("INSERT INTO plugins (id, status, permissions, config) VALUES ('messy','inactive','not-json','not-json')").run();
     const rt = new PluginRuntimeService();
     await rt.activate('messy'); // must not throw despite the garbage JSON
     expect(rt.isActive('messy')).toBe(true);