docs: add backend documentation files

Reference guides (Sequelize model methods, Swagger) and migration reports
from the MongoDB-to-MySQL and domain-based route structure migrations.

docs/MIGRATION_REPORT.md

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+                    API STRUCTURE MIGRATION REPORT
+═══════════════════════════════════════════════════════════════════════════════
+
+MIGRATION: v1/v2/v3 → Domain-Based (skydive/cms/herowars)
+
+OLD STRUCTURE:
+  └── routes/api/
+      ├── v1/ (Skydive routes)
+      ├── v2/ (CMS + misc routes)
+      └── v3/ (Hero Wars routes)
+
+NEW STRUCTURE:
+  └── routes/api/
+      ├── skydive/
+      ├── cms/
+      └── herowars/
+
+ROUTE MAPPINGS:
+  ✓ /api/v1/* → /api/skydive/*
+    - applications, aeronefs, canopies, dropzones, jumps, pages, profiles, qcm
+
+  ✓ /api/v2/* → /api/cms/*
+    - user, articles, comments, product, products, profiles, tags
+
+  ✓ /api/v3/* → /api/herowars/*
+    - clans, members, herowars (moved from v2)
+
+FILES CREATED/UPDATED:
+  ✓ src/routes/api/skydive/index.js
+  ✓ src/routes/api/cms/index.js
+  ✓ src/routes/api/herowars/index.js
+  ✓ src/routes/api/index.js
+  ✓ All route files copied to appropriate domains
+
+MANUAL STEPS REQUIRED:
+
+  1. Update Swagger/OpenAPI documentation:
+     - Review MIGRATION_SWAGGER_TODO.md
+     - Update path prefixes in src/swagger/*.yaml files
+     - Update paths in src/config/swagger.js if needed
+
+  2. Update app.js (if direct version references exist):
+     - Search for 'v1', 'v2', 'v3' strings
+     - Replace with domain names where appropriate
+
+  3. Update tests and client calls:
+     - Replace /api/v1/ with /api/skydive/
+     - Replace /api/v2/ with /api/cms/
+     - Replace /api/v3/ with /api/herowars/
+
+  4. Update any documentation:
+     - README.md API endpoints section
+     - API documentation and guides
+
+  5. Test all endpoints:
+     - Verify each domain routes work correctly
+     - Check authentication/authorization still functions
+     - Validate error handling
+
+OPTIONAL: Old v1/v2/v3 directories can be removed after verification:
+  rm -rf src/routes/api/v1
+  rm -rf src/routes/api/v2
+  rm -rf src/routes/api/v3
+
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