feat: implement configurable exponential backoff for webhook retries

[barry01-hash]

Closes #815

• Add migration 047_webhook_retry_backoff for per-endpoint retry config columns
• Add retryMaxAttempts, retryBaseDelayMs, retryMaxDelayMs, retryBackoffMultiplier, retryJitterRatio to webhook_endpoints
• Add computeRetryDelay helper with exponential backoff + jitter support
• Fix next_retry_at bug where RETRY_DELAYS[Math.min(0,0)] always used index 0
• Update createEndpoint/updateEndpoint to accept backoff configuration
• Update webhook delivery worker to use endpoint-specific retry policies
• Update mapToEndpoint to include new backoff fields
• Add WebhookRetryConfig type with safe defaults (7 retries, 1s base, 2x multiplier, 0.2 jitter, 1h max)
• Add validation constraints: maxAttempts 1-20, baseDelay >=100ms, multiplier 1-10, jitter 0-1
• Update outbox service to support backoff config in create/update

Description

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behaviour)
• Refactoring (no functional changes)
• Documentation update
• CI/CD change
• Dependency update

Related Issue

Closes #

Changes Made

Testing

• Unit tests pass locally (npm run test:unit)
• Integration tests pass locally (npm run test:integration)
• New tests added for new behaviour
• Manual testing completed — describe below if relevant

Manual test steps (if applicable):

Migration Changes

• No database migrations in this PR
• New migration file generated with npm run migrate:make
• Migration validated with npm run migrate:validate
• down() function tested locally
• No data loss in the rollback path

Documentation

• No documentation changes needed
• README.md updated (new commands, endpoints, or setup steps)
• Relevant docs/ file updated
• .env.example updated (new environment variables)
• Inline comments / JSDoc updated for changed functions
• backend/docs/API.md updated (new or changed endpoints)

Checklist

• Branch is up to date with main
• PR title follows Conventional Commits format (type(scope): summary)
• Code follows project style — linters pass (npm run lint, cargo clippy)
• Build succeeds (npm run build, cargo build --release)
• No console.log / println! left in production code
• No secrets, credentials, or .env files committed
• Self-review completed — I have read my own diff

CI Status

• Backend lint, build, and tests pass
• Frontend lint, build, and tests pass
• Contract format check, Clippy, and tests pass
• Security scan passes (no new vulnerabilities)
• Docker build succeeds

Screenshots

Breaking Changes

Additional Notes

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[barry01-hash]

Please merge

[Mosas2000]

Please fix the failed check

@barry01-hash

[Mosas2000]

@barry01-hash