refactor: reorganize project structure and update configurations

- Move migrations/ to database/schema/ for clarity on database schema definitions
- Move sqlc.yaml to internal/database/ to group with database code
- Move static/ to cmd/server/static/ to co-locate with server
- Update all configuration files and documentation
- Follow Go project conventions for better organization

internal/handlers/ebook.go

@@ -426,7 +426,7 @@ func (h *Handler) GetEbookRatings(c echo.Context) error {
 
 // ScanEbooksRequest represents the request for scanning ebooks
 type ScanEbooksRequest struct {
-	FolderPaths []string `json:"folder_paths" validate:"required,min=1"`
+	FolderPaths []string `json:"folder_paths,omitempty"`
 }
 
 // ScanEbooks handles POST /api/scanner/scan
@@ -435,12 +435,40 @@ func (h *Handler) ScanEbooks(c echo.Context) error {
 	if err := c.Bind(&req); err != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
 	}
-	if err := c.Validate(&req); err != nil {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
+
+	// Get user ID from JWT token
+	userID := c.Get("user_id").(string)
+	userUUID, err := uuid.Parse(userID)
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid user id"})
+	}
+
+	var folderPaths []string
+
+	// If folder paths provided in request, use them
+	// Otherwise, use user's saved folders
+	if len(req.FolderPaths) > 0 {
+		folderPaths = req.FolderPaths
+	} else {
+		// Get user's configured ebook folders
+		folders, err := h.db.GetUserEbookFolders(c.Request().Context(), pgtype.UUID{Bytes: userUUID, Valid: true})
+		if err != nil {
+			return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to get user folders: " + err.Error()})
+		}
+
+		if len(folders) == 0 {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "no folders configured for user"})
+		}
+
+		// Convert to folder paths
+		folderPaths = make([]string, len(folders))
+		for i, folder := range folders {
+			folderPaths[i] = folder.FolderPath
+		}
 	}
 
 	// Set the folder paths for scanning
-	if err := h.scanner.SetFolders(req.FolderPaths); err != nil {
+	if err := h.scanner.SetFolders(folderPaths); err != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid folder paths: " + err.Error()})
 	}
 
@@ -461,9 +489,6 @@ func (h *Handler) StartScanner(c echo.Context) error {
 	if err := c.Bind(&req); err != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
 	}
-	if err := c.Validate(&req); err != nil {
-		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
-	}
 
 	// Set the folder paths
 	if err := h.scanner.SetFolders(req.FolderPaths); err != nil {