fix: correctly handle HTTP suffix-byte-range requests (bytes=-N)

[dreamcreated]

Problem

Fixes #29

In app/services/media_server.py, the build_range_response function incorrectly handles suffix-byte-range requests (e.g. bytes=-500).

When a client sends Range: bytes=-500 (meaning "give me the last 500 bytes"), the old code splits on - to get ["", "500"]. Because parts[0] is empty, start was set to 0 and end to 500 — returning the first 501 bytes instead of the last 500.

This breaks clients that seek to the end of a file or read trailing metadata (e.g. MP4 moov atoms, ID3 tags).

Fix

Add a check for the suffix-byte-range format before the normal parse:

if not parts[0] and len(parts) > 1 and parts[1]:
    # Suffix-byte-range: bytes=-N means the last N bytes
    start = max(0, file_size - int(parts[1]))
    end = file_size - 1
else:
    start = int(parts[0]) if parts[0] else 0
    end = int(parts[1]) if len(parts) > 1 and parts[1] else file_size - 1

Tests

Verified with all four cases:

Range header	file_size	Expected	Result
bytes=-500	10000	9500–9999	✅
bytes=0-1023	10000	0–1023	✅
bytes=5000-	10000	5000–9999	✅
bytes=-99999	10000	0–9999 (clamped)	✅