feat(server): API relay gateway backend M0-M4

Gin + GORM + pure-Go SQLite. Users/auth (JWT), API key management with
quotas, proxy gateway with weighted channel failover and health checks,
usage/billing ledger, cross-protocol conversion (Anthropic Messages /
OpenAI Chat Completions / OpenAI Responses), and channel/model admin API.
Channels declare native API formats and auto-convert the rest.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/internal/proxy/convert_wrappers.go

@@ -0,0 +1,17 @@
+package proxy
+
+import (
+	"openteam/server/internal/proxy/convert"
+)
+
+// convertRequest translates a request body from the client protocol to the
+// channel provider's native format.
+func convertRequest(route Route, provider string, body []byte, upstreamModel string) ([]byte, error) {
+	return convert.Request(clientProto(route), provider, body, upstreamModel)
+}
+
+// convertResponse translates a non-stream upstream response back to the
+// client protocol.
+func convertResponse(route Route, provider string, body []byte) ([]byte, error) {
+	return convert.Response(clientProto(route), provider, body)
+}