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/001_trello_auth_spec.md

@@ -3,6 +3,7 @@
 **Feature:** US: Authenticate and Connect to Trello Account
 **Issue:** #1
 **Branch:** `feature/trello-auth`
+**Doc:** 001
 
 ## Overview
 

docs/backend/002_manage_boards_spec.md

@@ -3,6 +3,7 @@
 **Feature:** US: Manage Trello Boards
 **Issue:** #2
 **Branch:** `feature/manage-boards`
+**Doc:** 002
 
 ## Overview
 

docs/backend/003_manage_lists_spec.md

@@ -3,6 +3,7 @@
 **Feature:** US: Manage Trello Lists
 **Issue:** #3
 **Branch:** `feature/manage-lists`
+**Doc:** 003
 
 ## Overview
 

docs/backend/004_manage_cards_spec.md

@@ -3,6 +3,7 @@
 **Feature:** US: Manage Trello Cards
 **Issue:** #4
 **Branch:** `feature/manage-cards`
+**Doc:** 004
 
 ## Overview