Second-pass review fixes: kill last hardcoded creds, wire get_services, dedup

Verification audit (re-run of the 6 skill lenses) confirmed the prior fixes hold
and surfaced a few misses:

Security (HIGH):
- search.py _check_smart_redirect still opened a raw pymysql connection with
  root/rootpassword (reachable on any 9-digit SSO query). Now uses the shared
  env-backed employee_connection helper.
- Deleted dead shopdb/core/services/employee_service.py (zero importers; carried
  another root/rootpassword literal). No hardcoded credentials remain in app
  logic; config.py dev defaults stay gated by ProductionConfig.validate.

Dead hook:
- get_services was implemented by the printers plugin but had no consumer (docs
  claimed otherwise). Added PluginManager.get_service(name) that resolves a
  service from enabled plugins; updated PLUGIN-HOOKS.md.

Tests:
- search disabled-plugin exclusion (the high-value gap): enabled plugin's
  hostname appears, disabled plugin's hostname drops out (searched by a hostname
  distinct from assetnumber so only the gated domain can match).
- get_service consumer test (unknown name -> None).

Simplify:
- Extract the triplicated GE_LOGO_SVG + loadLogo + drawLogoOverlay into shared
  frontend/src/views/print/qrLogo.js (renderQrDataUrl); both QR views use it.
- applications.py: lift the misplaced pagination import to the top; drop unused
  Computer unpacking in the 3 endpoints that only touch ComputerInstalledApp.

154 tests pass, naming/style green, app boots, QR render verified.

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

docs/PLUGIN-HOOKS.md

@@ -140,7 +140,9 @@ class ComputersPlugin(BasePlugin):
 
 ### `get_services() -> Dict[str, Type]`
 
-Returns a dict of service-name to service-class. Other plugins can request services via the plugin manager.
+Returns a dict of service-name to service-class. Another plugin obtains one via
+`plugin_manager.get_service('<name>')`, which searches enabled plugins and
+returns the registered class/factory (or None).
 
 ```python
 from .services import ZabbixService