Complete GORM to sqlc migration

- Remove GORM dependency, use sqlc for type-safe SQL queries
- Update all handlers to use sqlc patterns (context, value types)
- Fix N+1 query problem in domain listing with JOIN query
- Enable SQL query logging in debug mode (USE_EMBEDDED=false)
- Add comprehensive comments for non-Go developers

backend/internal/api/handlers/stats.go

@@ -21,19 +21,21 @@ type Stats struct {
 }
 
 func (h *StatsHandler) Get(c *gin.Context) {
-	domains, err := h.db.GetAllDomains()
+	ctx := c.Request.Context()
+
+	domains, err := h.db.GetAllDomains(ctx)
 	if err != nil {
-		domains = []db.DomainStats{}
+		domains = nil
 	}
 
-	users, err := h.db.GetAllMailUsers()
+	users, err := h.db.GetAllMailUsers(ctx)
 	if err != nil {
-		users = []db.VirtualUser{}
+		users = nil
 	}
 
-	aliases, err := h.db.GetAllAliases()
+	aliases, err := h.db.GetAllAliases(ctx)
 	if err != nil {
-		aliases = []db.AliasWithDomain{}
+		aliases = nil
 	}
 
 	stats := Stats{