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>

admin-system/renovate.yaml

@@ -73,16 +73,6 @@ data:
           "datasourceTemplate": "docker",
           "packageNameTemplate": "linuxserver/{{depName}}",
           "versioningTemplate": "regex:^version-(?<major>\\d+)\\.(?<minor>\\d+)\\.(?<patch>\\d+)\\.(?<build>\\d+)$"
-        },
-        {
-          "description": "umami: the docker image tag is `postgresql-vX.Y.Z` (the PostgreSQL-flavored variant). Default docker versioning rejects the prefix. Same fix as termix/servarr: regex versioning parses the prefixed value; ghcr.io tag list is filtered to the postgresql-v* track only.",
-          "customType": "regex",
-          "managerFilePatterns": ["/felhom-system/umami\\.ya?ml$/"],
-          "matchStrings": [
-            "image:\\s+(?<depName>ghcr\\.io/umami-software/umami):(?<currentValue>postgresql-v\\d+\\.\\d+\\.\\d+)"
-          ],
-          "datasourceTemplate": "docker",
-          "versioningTemplate": "regex:^postgresql-v(?<major>\\d+)\\.(?<minor>\\d+)\\.(?<patch>\\d+)$"
         }
       ],
       "packageRules": [
@@ -160,11 +150,6 @@ data:
           ],
           "enabled": false
         },
-        {
-          "description": "umami: same disable pattern. customManager handles extraction; kubernetes-manager would silently skip `postgresql-vX.Y.Z`.",
-          "matchManagers": ["kubernetes"],
-          "matchPackageNames": ["ghcr.io/umami-software/umami"],
-          "enabled": false
         }
       ],
       "labels": ["renovate"]