feat: automate nightly database backups

[emirykl]

Summary

• add an opt-in nightly PostgreSQL backup service scheduled for 02:00 UTC
• validate custom-format dumps with pg_restore and upload them to S3-compatible storage with SHA-256 checksums
• track backup size, dump duration, total duration, object key, and run outcome in structured logs
• rotate expired backup objects using configurable retention and paginated S3 listing
• prevent overlapping runs and clean up temporary dump files
• document placeholder backup configuration and add focused service tests

Test evidence

• npm test -- --runInBand --no-watchman src/modules/backups/db-backup.service.spec.ts — passed, 4 tests
• focused TypeScript no-emit compilation — passed
• Prettier check — passed
• git diff --check — passed
• broad Jest run: the backup suite passed; 12 existing suites could not compile because the generated Prisma client is absent
• full build encountered the same missing Prisma client; generation requires access to a sandbox-denied user cache
• ESLint could not run because the repository contains no ESLint configuration

Fixes #58

Verification

• PASS: focused Jest suite (4 tests)
• PASS: focused TypeScript no-emit compilation
• PASS: Prettier check
• PASS: git diff --check
• Broad Jest: backup suite passed; 12 existing suites failed because the Prisma client is not generated
• Build: could not complete because Prisma generation requires writing to a sandbox-denied user cache
• Lint: repository has no ESLint configuration

Diagnostics

Recovered command failures: /bin/zsh -lc 'npm run build && npx eslint "src/modules/backups/**/*.ts" src/app.module.ts' (1), /bin/zsh -lc 'npx prisma generate && npm run build && npx eslint "src/modules/backups/**/*.ts" src/app.module.ts' (1), /bin/zsh -lc 'mkdir -p /tmp/hamplard-prisma-cache && XDG_CACHE_HOME=/tmp/hamplard-prisma-cache npx prisma generate && npm run build && npx eslint "src/modules/backups/**/*.ts" src/app.module.ts' (1), /bin/zsh -lc 'mkdir -p /tmp/hamplard-prisma-cache && CACHE_DIR=/tmp/hamplard-prisma-cache npx prisma generate && npm run build && npx eslint "src/modules/backups/**/*.ts" src/app.module.ts' (1), /bin/zsh -lc 'npm test -- --runInBand --no-watchman' (1)

[SYLVIANNORUKA]

@emirykl Pls resolve conflict