Add tags tree for notes.

migrations/add_note_folder_table.sql

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+-- Create note_folder table for tracking folders independently of notes
+CREATE TABLE IF NOT EXISTS note_folder (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100) NOT NULL,
+    path VARCHAR(500) NOT NULL,
+    parent_path VARCHAR(500),
+    description TEXT,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    created_by_id INTEGER NOT NULL REFERENCES "user"(id),
+    company_id INTEGER NOT NULL REFERENCES company(id),
+    CONSTRAINT uq_folder_path_company UNIQUE (path, company_id)
+);
+
+-- Create indexes for better performance
+CREATE INDEX IF NOT EXISTS idx_note_folder_company ON note_folder(company_id);
+CREATE INDEX IF NOT EXISTS idx_note_folder_parent_path ON note_folder(parent_path);
+CREATE INDEX IF NOT EXISTS idx_note_folder_created_by ON note_folder(created_by_id);