Update:added documentation folder and added all documentation required

docs/DATABASE_DESIGN.md

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+# Database Design Documentation
+
+## Overview
+This project uses SQLite as a local-first, single-user relational database engine via the `better-sqlite3` driver for Node.js.
+
+## Table Schema: `tasks`
+
+```sql
+CREATE TABLE IF NOT EXISTS tasks (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    title TEXT NOT NULL,
+    description TEXT,
+    due_date TEXT NOT NULL,
+    topic TEXT NOT NULL,
+    status TEXT CHECK(status IN ('Todo','In-Progress','Complete')) NOT NULL DEFAULT 'Todo',
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    is_archived BOOLEAN NOT NULL DEFAULT 0
+);
+```
+
+## Field Specifications
+
+| Column Name | Data Type | Constraints | Description |
+| :--- | :--- | :--- | :--- |
+| `id` | `INTEGER` | `PRIMARY KEY AUTOINCREMENT` | Unique identifier for each task record. |
+| `title` | `TEXT` | `NOT NULL` | Short summary title of the task. |
+| `description` | `TEXT` | Optional | Extended notes or detailed instructions for the task. |
+| `due_date` | `TEXT` | `NOT NULL` | Due date formatted as ISO string (`YYYY-MM-DD`). |
+| `topic` | `TEXT` | `NOT NULL` | Categorical topic tag (e.g. `SDP`, `Personal`). |
+| `status` | `TEXT` | `CHECK(status IN ('Todo','In-Progress','Complete'))` | Current lifecycle state of the task. Default is `'Todo'`. |
+| `created_at` | `TIMESTAMP` | `DEFAULT CURRENT_TIMESTAMP` | System timestamp recorded on task creation. |
+| `is_archived` | `BOOLEAN` | `DEFAULT 0` | Soft-deletion flag. `0` indicates active, `1` indicates archived. |
+
+## Schema Design Decisions
+
+1. Soft-Deletion (Archiving): Tasks are never permanently deleted from the database. Setting `is_archived = 1` retains all historical data while hiding archived tasks from active views.
+2. Dynamic Overdue State: The database schema explicitly excludes an `is_overdue` column. Storing `is_overdue` in a database column creates stale data when dates pass. Overdue status is dynamically computed at read-time by comparing `due_date` against the current date for non-completed tasks.