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

database/schema/005_create_user_ebook_folders.up.sql

@@ -0,0 +1,11 @@
+-- Create user_ebook_folders table for multiple folders per user
+CREATE TABLE user_ebook_folders (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    folder_path VARCHAR(500) NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    UNIQUE(user_id, folder_path)
+);
+
+-- Create index for faster lookups
+CREATE INDEX idx_user_ebook_folders_user_id ON user_ebook_folders(user_id);