Feature: Shared lists & Invite Code invitation mechanism

- Backend:
  - Migration 000002: add invite_code column to lists & list_members auto-population
  - ListStore: GetLists/GetList check owner_id & list_members; add GetInviteCode & JoinByInviteCode
  - HTTP API: add POST /api/lists/{id}/invite & POST /api/lists/join
- Android App:
  - DTOs & MitbringslApi: add JoinListRequestDto, InviteCodeResponseDto & endpoints
  - ListsScreen & ViewModel: add 'Liste beitreten' action button & dialog for entering invite code
  - ListDetailScreen & ViewModel: add 'Liste teilen' action icon in top bar with System Share Sheet
- ADB reinstall & launch verified ✅

backend/migrations/000002_add_invite_code.down.sql

@@ -0,0 +1 @@
+ALTER TABLE lists DROP COLUMN IF EXISTS invite_code;

backend/migrations/000002_add_invite_code.up.sql

@@ -0,0 +1,13 @@
+-- Add invite_code to lists table for sharing
+ALTER TABLE lists ADD COLUMN IF NOT EXISTS invite_code TEXT UNIQUE;
+
+-- Generate random 8-character invite code for lists that don't have one
+UPDATE lists
+SET invite_code = UPPER(SUBSTRING(MD5(RANDOM()::TEXT) FROM 1 FOR 8))
+WHERE invite_code IS NULL;
+
+-- Automatically insert owner into list_members on list creation if missing
+INSERT INTO list_members (list_id, user_id, role)
+SELECT id, owner_id, 'owner'
+FROM lists
+ON CONFLICT (list_id, user_id) DO NOTHING;