feat: Anthropic provider/inbound + embeddings

- Anthropic outbound provider: /v1/messages, system extraction, x-api-key +
  anthropic-version; streaming translates Anthropic SSE -> unified OpenAI wire format
- Anthropic inbound adapter (POST /v1/messages): non-streaming JSON + streaming SSE
  (OpenAI -> Anthropic event translation)
- Embeddings: POST /v1/embeddings inbound + Provider::embed(); embedding models
  resolvable from the pool (registry no longer skips them)
- protocols mounted always, gated on live flags; admin meta marks them implemented

src/providers/mod.rs

@@ -57,5 +57,13 @@ pub trait Provider: Send + Sync {
         )))
     }
 
-    // TODO(v0.2): async fn embed(...) -> Result<EmbedResponse>.
+    /// Embeddings call. `input` is the OpenAI `input` field (string or array of
+    /// strings); returns the provider's OpenAI-style embeddings response body.
+    /// Default: not supported (override per provider).
+    async fn embed(&self, _input: serde_json::Value) -> crate::error::Result<serde_json::Value> {
+        Err(GatewayError::UpstreamUnavailable(format!(
+            "embeddings are not supported by provider '{}'",
+            self.id()
+        )))
+    }
 }