fix(gallery): make opus a meta backend so the platform variant is auto-selected (#9813)

The realtime/WebRTC path loads the "opus" codec backend by name, but on
macOS arm64 only "metal-opus" is installable, so Load("opus") failed with
"opus backend not available".

The root cause: unlike llama-cpp and whisper, the opus entry was a concrete
CPU backend (it carried a uri and no capabilities map) rather than a meta
backend, so nothing mapped "opus" to the platform-appropriate variant.

Restructure opus to mirror llama-cpp/whisper: "opus" becomes a meta backend
with a capabilities map (default -> cpu-opus, metal -> metal-opus) and no
uri; the CPU image moves to a new "cpu-opus" concrete (and its dev variant
to "cpu-opus-development"). Installing "opus" now resolves to metal-opus on
Apple Silicon and cpu-opus elsewhere, and Load("opus") works on every
platform via the meta pointer - so the realtime endpoint needs no special
casing. This reverts the realtime_webrtc.go resolution helper from the
earlier approach in favor of the gallery-level fix.

Assisted-by: claude:claude-opus-4-8 [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

Author: mudler

backend/index.yaml

@@ -1163,11 +1163,11 @@
 - &opus
   name: "opus"
   alias: "opus"
-  uri: "quay.io/go-skynet/local-ai-backends:latest-cpu-opus"
+  capabilities:
+    default: "cpu-opus"
+    metal: "metal-opus"
   urls:
     - https://opus-codec.org/
-  mirrors:
-    - localai/localai-backends:latest-cpu-opus
   license: BSD-3-Clause
   description: |
     Opus audio codec backend for encoding and decoding audio.
@@ -1177,7 +1177,6 @@
     - opus
     - WebRTC
     - realtime
-    - CPU
 - &silero-vad
   name: "silero-vad"
   uri: "quay.io/go-skynet/local-ai-backends:latest-cpu-silero-vad"
@@ -1603,7 +1602,12 @@
   mirrors:
     - localai/localai-backends:master-metal-darwin-arm64-local-store
 - !!merge <<: *opus
-  name: "opus-development"
+  name: "cpu-opus"
+  uri: "quay.io/go-skynet/local-ai-backends:latest-cpu-opus"
+  mirrors:
+    - localai/localai-backends:latest-cpu-opus
+- !!merge <<: *opus
+  name: "cpu-opus-development"
   uri: "quay.io/go-skynet/local-ai-backends:master-cpu-opus"
   mirrors:
     - localai/localai-backends:master-cpu-opus