docs: rename spec docs to migration-style numbering

Rename docs/backend/*.md to Django-migration-style naming:
  trello-auth-spec.md    → 001_trello_auth_spec.md    (Issue #1)
  manage-boards-spec.md  → 002_manage_boards_spec.md  (Issue #2)
  manage-lists-spec.md   → 003_manage_lists_spec.md   (Issue #3)
  manage-cards-spec.md   → 004_manage_cards_spec.md   (Issue #4)

Each doc now includes a **Doc:** field with its sequence number.

docs/backend/004_manage_cards_spec.md

@@ -0,0 +1,163 @@
+# Trello Plugin — Manage Trello Cards
+
+**Feature:** US: Manage Trello Cards
+**Issue:** #4
+**Branch:** `feature/manage-cards`
+**Doc:** 004
+
+## Overview
+
+Extends the Trello plugin with full card management capabilities: create, view, update, move, archive, assign members, comments, and checklist management.
+
+## Tools
+
+### `trello_create_card`
+Create a new card on a selected list.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `name` | string | Yes | Card title |
+| `list_id` | string | Yes | List ID |
+| `desc` | string | No | Card description |
+| `due` | string | No | Due date (ISO 8601) |
+
+**Returns:** Card ID, name, URL, and short URL.
+
+### `trello_card_details`
+View detailed information about a card.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+
+**Returns:** Card details including title, description, due date, members, checklists, comments, URL.
+
+### `trello_update_card`
+Update a card's core attributes (title, description, due date).
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `name` | string | No | New title |
+| `desc` | string | No | New description |
+| `due` | string | No | New due date (ISO 8601) |
+
+**Returns:** Updated card details.
+
+### `trello_move_card`
+Move a card to a different list.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `list_id` | string | Yes | Target list ID |
+
+**Returns:** Success message with new list location.
+
+### `trello_archive_card`
+Archive a card.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+
+**Returns:** Success message.
+
+### `trello_assign_member`
+Add a member to a card.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `member_id` | string | Yes | Trello member ID |
+
+**Returns:** Success message.
+
+### `trello_remove_member`
+Remove a member from a card.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `member_id` | string | Yes | Trello member ID |
+
+**Returns:** Success message.
+
+### `trello_add_comment`
+Add a comment to a card.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `text` | string | Yes | Comment text |
+
+**Returns:** Comment ID and success.
+
+### `trello_delete_comment`
+Delete a comment from a card.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `comment_id` | string | Yes | Comment/action ID |
+
+**Returns:** Success message.
+
+### `trello_add_checklist_item`
+Add a checklist item to a card.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `name` | string | Yes | Checklist item text |
+| `checklist_id` | string | No | Specific checklist ID (if card has multiple) |
+
+**Returns:** Checklist item details.
+
+### `trello_toggle_checklist_item`
+Mark a checklist item as complete or incomplete.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `item_id` | string | Yes | Checklist item ID |
+| `checked` | bool | Yes | True = complete, False = incomplete |
+
+**Returns:** Updated item state.
+
+### `trello_delete_checklist_item`
+Remove a checklist item.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `card_id` | string | Yes | Card ID |
+| `item_id` | string | Yes | Checklist item ID |
+
+**Returns:** Success message.
+
+## Trello API Endpoints
+
+| Purpose | Method | Endpoint |
+|---------|--------|----------|
+| Create card | POST | `/1/cards` |
+| Get card | GET | `/1/cards/{id}` |
+| Update card | PUT | `/1/cards/{id}` |
+| Add member | POST | `/1/cards/{id}/idMembers` |
+| Remove member | DELETE | `/1/cards/{id}/idMembers/{memberId}` |
+| Add comment | POST | `/1/cards/{id}/actions/comments` |
+| Delete comment | DELETE | `/1/cards/{id}/actions/{commentId}/comments` |
+| Add checklist item | POST | `/1/cards/{id}/checklistItems` |
+| Update checklist item | PUT | `/1/cards/{id}/checklistItem/{itemId}/state` |
+| Delete checklist item | DELETE | `/1/cards/{id}/checklistItems/{itemId}`