Apply skill-driven review fixes: security, hook isolation, tests, docs

Addresses findings from a 6-lens review against the project skills
(defining-asset-contract, enforcing-plugin-contract, hardening-flask-config,
integrating-plugin-hooks, pinning-flask-behavior, simplifying-python).

Security (hardening-flask-config):
- Load per-plugin COLLECTOR_API_KEY_<PLUGIN> from env in create_app. from_object
  only copies class attributes, so per-plugin keys (ADR-006) were dead in real
  deploys and silently fell back to the shared key.
- EMPLOYEE_DB_USER/PASSWORD no longer default to root/rootpassword (no safe
  default for a secret; unset fails loud). Documented in .env.example + DEPLOY.md.
- COLLECTOR_API_KEY + per-plugin + EMPLOYEE_DB_* added to .env.example/DEPLOY.md.

Hook isolation (integrating-plugin-hooks):
- collector _collector_plugins and dashboard get_navigation now re-raise in
  dev/test and log+isolate in prod, instead of silently swallowing a broken
  plugin hook.

Plugin loader (enforcing-plugin-contract):
- enable_plugin/install_plugin read dependencies+version from the manifest
  instead of instantiating the plugin class.
- _register_plugin_components rejects a second plugin claiming an already-used
  api_prefix (reset per app in init_app).

Tests (pinning-flask-behavior):
- test_identifiers.py: gauge/maintenance round-trip on computer/printer/network
  create+update; per-type seed yields the 12 identifier keys.
- contract tests for apply_collector_payload presence + schema-declarers-implement.
- security tests for per-plugin key env loading + no employee-db password default.

Docs/contract sync (defining-asset-contract):
- PLUGIN-HOOKS.md documents apply_collector_payload; stale 0.2.0 -> 0.3.0.
- ADR-006 documents apply_collector_payload + single-dispatch rationale.
- ADR-001 enumerates the expanded shopdb.api import surface.

Simplify (simplifying-python):
- De-duplicate the 21-entry settings defaults: shared build_default_settings()
  used by both the /settings/seed route and the CLI (were drifting copies).
- Remove dead AssetStatus import + redundant AssetType local import in computers
  plugin; comment the statusid=1 collector default.

153 tests pass (was 145), naming/style green.

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

.env.example

@@ -58,3 +58,12 @@ ZABBIX_TOKEN=
 # COLLECTOR_API_KEY_<PLUGINNAME> first, then COLLECTOR_API_KEY as fallback.
 # COLLECTOR_API_KEY=
 # COLLECTOR_API_KEY_COMPUTERS=
+
+# ---- Employee directory database (optional, read-only) ----
+# Separate HR/employee lookup DB consumed by the notifications plugin and the
+# public shopfloor kiosks. Leave unset if the feature is not used; there is no
+# safe default for the password, so an unset password fails loud.
+# EMPLOYEE_DB_HOST=
+# EMPLOYEE_DB_USER=
+# EMPLOYEE_DB_PASSWORD=
+# EMPLOYEE_DB_NAME=wjf_employees