Update:added documentation folder and added all documentation required

docs/RELATIONSHIPS.md

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+# Entity Relationships and Data Lifecycle
+
+## System Architecture Model
+
+The application operates as a single-table relational model optimized for local-first desktop usage.
+
+```mermaid
+classDiagram
+    class Task {
+        +int id
+        +string title
+        +string description
+        +string due_date
+        +string topic
+        +TaskStatus status
+        +boolean is_archived
+        +string created_at
+        +boolean computeIsOverdue()
+    }
+
+    class TaskStatus {
+        <<enumeration>>
+        Todo
+        In-Progress
+        Complete
+    }
+
+    Task "1" -- "1" TaskStatus : holds
+```
+
+## State Transitions & Task Lifecycle
+
+A task record moves through defined state transitions:
+
+```mermaid
+stateDiagram-v2
+    [*] --> Todo : Task Created
+    Todo --> InProgress : User updates status
+    InProgress --> Complete : Task completed
+    Complete --> InProgress : Reopened
+    
+    Todo --> Archived : Soft Deleted
+    InProgress --> Archived : Soft Deleted
+    Complete --> Archived : Soft Deleted
+
+    Archived --> Todo : Restored (Unarchived)
+```
+
+## Overdue Derivation Logic
+
+Overdue state is derived dynamically at query runtime using the following logic:
+
+- Condition 1: `status !== 'Complete'`
+- Condition 2: `due_date < current_date` (ISO format `YYYY-MM-DD`)
+
+If both conditions are met, `is_overdue` evaluates to `true`. Completed or archived tasks are never marked as overdue.