Add exponential backoff retry for Overpass API timeouts

[Copilot]

Overpass API calls fail on transient 504 Gateway Timeout and 429 Rate Limit errors without retry, causing unnecessary data collection failures.

Changes

• Retry logic with exponential backoff: Added withRetry<T> wrapper in collect/api/overpass.ts that attempts requests 3 times (initial + 2 retries) with 1s, 2s backoff delays
• Selective retry: Only retries on OverpassGatewayTimeoutError (504) and OverpassRateLimitError (429); other errors fail fast
• Transparent integration: Both overpassSimpleQuery() and overpassRawQuery() automatically benefit through callApi()

Implementation

async function withRetry<T>(function_: () => Promise<T>, maxRetries: number = 2): Promise<T> {
    for (let attempt = 0; attempt <= maxRetries; attempt += 1) {
        try {
            return await function_();
        } catch (error) {
            const isRetriableError = error instanceof OverpassGatewayTimeoutError 
                || error instanceof OverpassRateLimitError;
            
            if (!isRetriableError || attempt === maxRetries) {
                throw error;
            }
            
            await delay((2 ** attempt) * 1000); // Exponential backoff
        }
    }
}

Test coverage includes retry success scenarios, backoff timing verification, retry exhaustion, and non-retriable error handling.

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