feat(backend): add retry logic with exponential backoff (M5-04)

[liamstevens]

Summary

• Add internal/backend/retry.go with retry logic for transient failures
• Exponential backoff with configurable parameters and jitter

Retry Configuration

Setting	Default	Description
MaxAttempts	3	Maximum attempts (including initial)
InitialWait	1s	Initial wait before first retry
MaxWait	30s	Maximum wait between retries
Multiplier	2.0	Backoff multiplier
Jitter	0.1	Random jitter fraction (0-1)

Error Classification

Retryable Errors

• Network errors (connection refused, timeout, DNS)
• Server errors (500, 502, 503, 504)
• Unknown errors (conservative default)

Non-Retryable Errors

• Client errors (400, 401, 403, 404, 409)
• Authentication failures
• Resource not found

Usage

import "github.com/valent-au/git-los/internal/backend"

// With default config (3 attempts, 1s initial, 2x backoff)
err := backend.RetryFunc(ctx, func() error {
    return doSomething()
})

// With custom config
err := backend.Retry(ctx, backend.RetryConfig{
    MaxAttempts: 5,
    InitialWait: 500 * time.Millisecond,
    MaxWait:     10 * time.Second,
}, func() error {
    return doSomething()
})

Test plan

• go test ./internal/backend/... - All retry tests pass
• go test ./... - Full test suite passes
• ~/go/bin/golangci-lint run - Lint clean
• go build ./cmd/git-los - Builds successfully

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