fix(plugins): complete steward rename across bundled plugins; clear lint debt

The fabledscryer->steward rename had only ever reached host_agent. The other
five bundled plugins (http, snmp, traefik, unifi, docker) still imported
`from fabledscryer.*` (package no longer exists) and read FABLEDSCRYER_* env
vars — so every one of them was broken at import since the original rebrand.
CI stayed green only because none are enabled by default and migrations don't
import plugin modules. Now that they version in-tree, complete the rename:
- fabledscryer.* -> steward.* imports across all five plugins
- FABLEDSCRYER_* -> STEWARD_* in plugin migration env.py files
- author/repository/homepage + user-facing 'Fabled Scryer' strings -> Steward
- snmp/scheduler.py: also drop dead `now`/datetime; record_metric from steward

Adds tests/test_no_legacy_names.py — fails if 'scryer'/'roundtable' ever
reappear in shipped code (the drift bit twice; this stops a third time).

Also clears pre-existing ruff lint debt (unused imports, semicolon statements,
mid-file import) surfaced by the new lint lane.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/unifi/plugin.yaml

@@ -1,11 +1,11 @@
 name: unifi
 version: "1.0.0"
 description: "UniFi Network controller integration — WAN health, devices, clients, DPI"
-author: "FabledScryer"
+author: "Steward"
 license: "MIT"
 min_app_version: "0.1.0"
-repository_url: "https://git.fabledsword.com/bvandeusen/FabledScryer-plugins"
-homepage: "https://git.fabledsword.com/bvandeusen/FabledScryer-plugins/src/branch/main/unifi"
+repository_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins"
+homepage: "https://git.fabledsword.com/bvandeusen/Steward-plugins/src/branch/main/unifi"
 tags:
   - network
   - unifi