cleanup: drop stale felhom-system copies + fix plex datasource + remove obsolete umami config

Three coordinated changes, all surfaced by the question "will Renovate
track the manually-bumped packages going forward":

1) Delete `felhom-system/` directory (4 files).
   These were never the source of truth -- the `felhom` ArgoCD app
   pulls from `felhom.eu`, path `manifests`. The copies in this repo
   fell out of sync over time and were misleading. Renovate was about
   to start opening DEAD PRs against them (the customManager below
   targeted `felhom-system/umami.yaml`). Removing the directory is the
   cleanest fix; manual bumps for the real felhom-system manifests go
   into the felhom.eu repo.

2) Fix plex inline `# renovate:` comment in helm/plex/values.yaml.
   It referenced `datasource=custom.plex` but no such customDatasource
   exists in the config -- Renovate would silently skip plex. Switched
   to the standard docker datasource with regex versioning that parses
   `1.X.Y.Z-<hash>` (4 segments + git short-hash suffix, same pattern
   approach as servarr and termix).

3) Remove the now-obsolete umami customManager + packageRule.
   The customManager was for the `postgresql-vX.Y.Z` tag form we've
   abandoned -- the real felhom.eu deployment is on `3.1.0` (plain
   semver). The packageRule disabled the kubernetes manager for the
   umami image to silence its failure on `postgresql-vX.Y.Z`; not
   needed since the default versioning handles `3.X.Y` fine. (Moot
   anyway since Renovate doesn't watch felhom.eu -- but cleanup
   reduces config noise.)

After this PR, Renovate's effective tracking:
  - servarr (sonarr/radarr/prowlarr)      -> YES (customManager)
  - plex                                   -> YES (inline comment, docker)
  - termix                                 -> YES (customManager)
  - umami / filebrowser in felhom.eu      -> NO (different repo, manual)
  - all standard semver/named tags in homelab-manifests -> YES (defaults)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

helm/plex/values.yaml

@@ -3,7 +3,7 @@ image:
   # -- The public dockerhub registry
   registry: index.docker.io
   repository: plexinc/pms-docker
-  # renovate: datasource=custom.plex depName=plex versioning=loose
+  # renovate: datasource=docker depName=plexinc/pms-docker versioning=regex:^(?<major>\d+)\.(?<minor>\d+)\.(?<patch>\d+)\.(?<build>\d+)-[a-f0-9]+$
   tag: "1.43.2.10687-563d026ea"
   sha: ""
   pullPolicy: IfNotPresent