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/003_manage_lists_spec.md

@@ -0,0 +1,89 @@
+# Trello Plugin — Manage Trello Lists
+
+**Feature:** US: Manage Trello Lists
+**Issue:** #3
+**Branch:** `feature/manage-lists`
+**Doc:** 003
+
+## Overview
+
+Extends the Trello plugin with list management capabilities: create, rename, archive, and reposition lists on a Trello board.
+
+## Design Decisions
+
+- **Lists require a board context** — Creating a list needs a board ID. Other operations (rename, archive, move) use the list's Trello ID which is globally unique.
+- **Position parameter** — Uses Trello's `pos` field which accepts `"top"`, `"bottom"`, or a positive number.
+
+## Tools
+
+### `trello_create_list`
+
+Create a new list on a board.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `name` | string | Yes | List name |
+| `board_id` | string | Yes | Board ID or name |
+| `pos` | string | No | Position: `"top"`, `"bottom"`, or number (default: `"bottom"`) |
+
+**Returns:**
+```json
+{"success": true, "list": {"id": "l1", "name": "My List", "id_board": "b1"}}
+```
+
+### `trello_rename_list`
+
+Rename an existing list.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `list_id` | string | Yes | List ID |
+| `name` | string | Yes | New name |
+
+**Returns:**
+```json
+{"success": true, "list": {"id": "l1", "name": "Renamed List"}}
+```
+
+### `trello_archive_list`
+
+Archive a list.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `list_id` | string | Yes | List ID |
+
+**Returns:**
+```json
+{"success": true, "message": "List 'My List' archived."}
+```
+
+### `trello_move_list`
+
+Move a list to a different position.
+
+**Parameters:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `list_id` | string | Yes | List ID |
+| `pos` | string | Yes | Position: `"top"`, `"bottom"`, or a number |
+
+**Returns:**
+```json
+{"success": true, "message": "List 'My List' moved."}
+```
+
+## Trello API Endpoints
+
+| Purpose | Method | Endpoint |
+|---------|--------|----------|
+| Create list | POST | `/1/lists` |
+| Update list (rename, archive, move) | PUT | `/1/lists/{id}` |
+
+## Error Handling
+
+- List not found → clear error message
+- Board not found when creating → propagate board lookup error