feat: migrate backend from rust to go

This commit replaces the Axum/Rust backend with a Gin/Go implementation. The original Rust code has been archived in the 'rust' branch.

internal/middleware/auth.go

@@ -0,0 +1,52 @@
+package middleware
+
+import (
+	"log"
+	"strings"
+
+	"llm-proxy/internal/db"
+	"llm-proxy/internal/models"
+
+	"github.com/gin-gonic/gin"
+)
+
+func AuthMiddleware(database *db.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		authHeader := c.GetHeader("Authorization")
+		if authHeader == "" {
+			c.Next()
+			return
+		}
+
+		token := strings.TrimPrefix(authHeader, "Bearer ")
+		if token == authHeader { // No "Bearer " prefix
+			c.Next()
+			return
+		}
+
+		// Try to resolve client from database
+		var clientID string
+		err := database.Get(&clientID, "UPDATE client_tokens SET last_used_at = CURRENT_TIMESTAMP WHERE token = ? AND is_active = 1 RETURNING client_id", token)
+		
+		if err == nil {
+			c.Set("auth", models.AuthInfo{
+				Token:    token,
+				ClientID: clientID,
+			})
+		} else {
+			// Fallback to token-prefix derivation (matches Rust behavior)
+			prefixLen := len(token)
+			if prefixLen > 8 {
+				prefixLen = 8
+			}
+			clientID = "client_" + token[:prefixLen]
+			c.Set("auth", models.AuthInfo{
+				Token:    token,
+				ClientID: clientID,
+			})
+			log.Printf("Token not found in DB, using fallback client ID: %s", clientID)
+		}
+
+		c.Next()
+	}
+}