Migrate transmission containers from Dockerfile to Dagger builds

Replace Dockerfiles with native container.py for both transmission and
transmission-exporter. Updates base images (Alpine 3.23, Python 3.14),
pins uv to 0.11.6 instead of :latest.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

service-versions.yaml

@@ -196,13 +196,13 @@ services:
 
   - name: transmission
     type: argocd
-    last-reviewed: 2026-03-04
+    last-reviewed: 2026-04-15
     current-version: "4.1.1-r1"
     upstream-source: https://github.com/transmission/transmission/releases
 
   - name: transmission-exporter
     type: argocd
-    last-reviewed: 2026-03-05
+    last-reviewed: 2026-04-15
     current-version: "1.0.1"
     upstream-source: null
     notes: Homegrown Python exporter, no upstream