Implement HTTP Range request support for getBlob endpoint

[DavidBuchanan314]

Implements #33

Add comprehensive range request support to com.atproto.sync.getBlob with optimized blob size calculation and selective part fetching. (via claude)

Changes:

• Add BLOB_PART_SIZE constant to static_config.py (64KB)
• Update blob upload to use centralized BLOB_PART_SIZE constant
• Implement range request handling using aiohttp's http_range property
• Support normal ranges (bytes=start-end), open-ended (bytes=start-), and suffix ranges (bytes=-count)
• Return 206 Partial Content with Content-Range header for ranges
• Return 416 Range Not Satisfiable for invalid ranges
• Add Accept-Ranges: bytes header to all responses
• Optimize blob size calculation: count parts + last part length instead of scanning all parts
• Optimize part fetching: only fetch required part indices from database

Tests:

• Add comprehensive range request tests covering all range types
• Add edge case tests for 64KB part boundary conditions
• Add zero-length blob test

[Copilot]

Pull Request Overview

This PR implements HTTP Range request support for the com.atproto.sync.getBlob endpoint, allowing clients to request specific byte ranges from blobs. This enables partial content downloads and better support for resumable downloads.

• Add centralized BLOB_PART_SIZE constant (64KB) to static configuration
• Implement comprehensive range request handling with optimized blob size calculation and selective part fetching
• Add extensive test coverage for various range request scenarios including edge cases and boundary conditions

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/millipds/static_config.py	Adds centralized BLOB_PART_SIZE constant (64KB) with documentation
src/millipds/atproto_repo.py	Updates blob upload to use the centralized BLOB_PART_SIZE constant
src/millipds/atproto_sync.py	Implements HTTP range request handling with optimized blob size calculation and selective part fetching
tests/test_blob_management.py	Adds comprehensive test coverage for range requests including boundary conditions and edge cases

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