fix: use deterministic hash for ETag generation

[awanawana]

Summary

Fixes #400

Python's built-in hash() function is salted by default (via PYTHONHASHSEED), which means it produces different values:

• Across different Python processes
• After service restart
• Between multiple backends behind a load balancer

This causes ETags to change unexpectedly, invalidating browser caches and making the cache mechanism effectively useless in distributed deployments.

Changes

Replace hash() with hashlib.md5() for ETag generation:

def _compute_etag(data: bytes) -> str:
    """Compute a deterministic ETag hash from cache data."""
    return hashlib.md5(data).hexdigest()

MD5 is chosen for speed since this is for caching (not security). The hash is consistent across processes and restarts.

Before/After

Before (unpredictable across processes):

ETag: W/-4586327899234  # Process 1
ETag: W/8923847234987   # Process 2 (same data, different hash)

After (deterministic):

ETag: W/d41d8cd98f00b204e9800998ecf8427e  # Process 1
ETag: W/d41d8cd98f00b204e9800998ecf8427e  # Process 2 (same data, same hash)

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