feat(storage): integrate Cloudflare R2 for object storage

[GRACENOBLE]

Summary

• Wires the existing R2 infrastructure layer end-to-end across backend, web, and mobile
• Adds POST /api/v1/storage/presign and DELETE /api/v1/storage/:key — both Firebase-auth-protected and gated on R2_ACCOUNT_ID being set
• Adds useUpload() hook on web for presign → direct-to-R2 PUT flow
• Adds R2UploadRepository on Android using OkHttp with injectable client for testing

Changes by layer

Backend

• usecase/storage.go — StorageService interface at the correct Clean Architecture layer
• infrastructure/storage/r2/storage.go — updated to return usecase.StorageService; adds NewWithHTTPClient for test injection
• Bootstrap wiring: App.StorageService, R2 Config fields, optional init block
• Two new authenticated handlers with swaggo annotations; Swagger docs regenerated
• .env.example updated with R2_ACCOUNT_ID, R2_ACCESS_KEY, R2_SECRET_KEY, R2_BUCKET, R2_PUBLIC_URL

Web

• lib/storage.ts — presign() and uploadToR2() utilities
• lib/useUpload.ts — useUpload() hook with isUploading, publicUrl, error, upload

Mobile

• storage/UploadRepository.kt — UploadRepository interface + R2UploadRepository impl
• okhttp-mockwebserver added as testImplementation

Docs

• New: backend/docs/storage.md, web/docs/storage.md, mobile/docs/storage.md
• Updated: environment.md, bootstrap.md
• Fixed stale: error-handling.md, routing.md, websocket.md

Test plan

• Backend: go test ./internal/transport/handlers/... ./internal/infrastructure/storage/... — handler unit tests + R2 package tests (custom RoundTripper mock)
• Web: pnpm test — 40 tests pass including 13 new storage/useUpload tests
• Mobile: .\gradlew.bat test — UploadRepositoryTest passes with MockWebServer
• Manual: set R2 env vars, call POST /api/v1/storage/presign, upload a file to the returned URL, verify it appears at the public URL
• Manual: call DELETE /api/v1/storage/:key, verify the object is removed

Closes #22

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ab8bdc9a-0121-422d-a1f4-28da8e1406db

📥 Commits

Reviewing files that changed from the base of the PR and between fe3b1af and d922b2c.

📒 Files selected for processing (33)

• backend/.env.example
• backend/docs/_index.md
• backend/docs/bootstrap.md
• backend/docs/environment.md
• backend/docs/error-handling.md
• backend/docs/routing.md
• backend/docs/storage.md
• backend/docs/swagger/docs.go
• backend/docs/swagger/swagger.json
• backend/docs/swagger/swagger.yaml
• backend/docs/websocket.md
• backend/internal/bootstrap/bootstrap.go
• backend/internal/infrastructure/storage/r2/storage.go
• backend/internal/infrastructure/storage/r2/storage_test.go
• backend/internal/server/server.go
• backend/internal/transport/handlers/handler.go
• backend/internal/transport/handlers/health_handler_test.go
• backend/internal/transport/handlers/routes.go
• backend/internal/transport/handlers/storage_handler.go
• backend/internal/transport/handlers/storage_handler_test.go
• backend/internal/usecase/storage.go
• mobile/app/build.gradle.kts
• mobile/app/src/main/java/com/company/template/storage/UploadRepository.kt
• mobile/app/src/test/java/com/company/template/storage/UploadRepositoryTest.kt
• mobile/docs/_index.md
• mobile/docs/storage.md
• mobile/gradle/libs.versions.toml
• web/docs/_index.md
• web/docs/storage.md
• web/lib/storage.test.ts
• web/lib/storage.ts
• web/lib/useUpload.test.ts
• web/lib/useUpload.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 22-feat-cloudflare-r2-storage

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