feat(backend): add static asset serving support (closes #217)

[otobongdev]

Summary

Adds backend support for serving static and uploaded assets via a new NestJS AssetsModule, plus a read-only static asset directory mounted from disk. Closes #217.

What's new

• New AssetsModule under BackendAcademy/src/assets/:
  • assets.module.ts, assets.controller.ts, assets.service.ts
  • interfaces/asset.interface.ts (Asset, AssetListResponse, AssetSortOrder)
  • dto/upload-asset.dto.ts (UploadAssetDto with optional name and description text fields validated alongside the multipart upload)
  • Specs: assets.controller.spec.ts, assets.service.spec.ts
• Wiring:
  • app.module.ts registers AssetsModule
  • main.ts is now typed as NestExpressApplication and mounts ASSETS_STATIC_DIR (default ./public) under the URL prefix /static/ via useStaticAssets, with mkdirSync fallback so a fresh clone boots.
  • config/config.module.ts adds Joi-validated env vars: ASSETS_UPLOAD_DIR, ASSETS_MAX_SIZE_MB, ASSETS_BASE_URL, ASSETS_STATIC_DIR.
  • .env.example documents the new env vars.
• Dependencies:
  • multer ^1.4.5-lts.1 (now a direct dep so import { memoryStorage } from 'multer' resolves under pnpm strict mode)
  • @types/multer ^1.4.12 and typescript-eslint ^7.18.0 under devDependencies
• Misc:
  • Removed an extra trailing } in BackendAcademy/src/social/social.service.ts that was blocking TypeScript compilation.

REST surface (URI prefix /api/v1/ thanks to global versioning)

Method	Path	Description
GET	/assets	List assets (?sort=newest|oldest|name, default newest)
GET	/assets/:id	Fetch asset metadata
GET	/assets/:id/download	Stream content back to client with stored Content-Type, length, and Content-Disposition
POST	/assets	Multipart upload (field file); optional name/description validated against UploadAssetDto
DELETE	/assets/:id	Remove asset metadata and the underlying file

Read-only prebuilt static files under ASSETS_STATIC_DIR are served at /static/*.

Behavior

• Metadata is held in an in-memory Map<id, Asset>; the file body is persisted at ASSETS_UPLOAD_DIR (default ./data/uploads).
• Upload guard rails: size cap (default 10 MB, configurable via ASSETS_MAX_SIZE_MB), MIME allowlist (image/*, video/*, audio/*, application/pdf, text/*).
• Filenames in the upload directory embed the asset UUID and are sanitised so the resolved path cannot escape ASSETS_UPLOAD_DIR.
• Orphan files in the upload directory are pruned on graceful shutdown via OnModuleDestroy.
• Swagger annotations on every endpoint so the change shows up in /api/docs.

Tests

• pnpm test in BackendAcademy reports 113 of 117 tests passing.
• The 4 failures (src/rewards/rewards.service.spec.ts \u2014 nested test definition; src/rewards/streak.service.spec.ts \u2014 stale getStreak / checkIn assertions) are pre-existing on origin/main and unrelated to this change. They are documented here for transparency; fixing them is out of scope for issue Add static asset serving support #217.
• pnpm build (nest build) compiles cleanly.
• Note: pnpm lint currently fails on origin/main because the repo uses eslint.config.ts (flat config) with eslint@^8.0.0 declared in devDependencies. Flat config is supported by eslint 9 or by eslint 8 with ESLINT_USE_FLAT_CONFIG=true; align versions in a follow-up PR.

Configuration

# Static & uploaded assets
ASSETS_UPLOAD_DIR=./data/uploads
ASSETS_MAX_SIZE_MB=10
ASSETS_BASE_URL=/api/v1/assets
ASSETS_STATIC_DIR=./public

Closes #217

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