fix: retry OBS downloads on 404 and handle media failures gracefully

[clins1994]

Summary

• Retry OBS media downloads on 404 responses (LINE returns 404 transiently for recently uploaded media)
• Gracefully handle media download failures instead of breaking the entire message flow

Please see: https://github.com/clins1994/matrix-line-messenger/pull/2 for more information

Closes #71

Problem (before)

flowchart TD
    A[Bundle of photos sent on LINE] --> B[Bridge receives multiple messages]
    B --> C[Download media from OBS for each]
    
    C --> D{OBS response?}
    D -->|200 OK| E[Media bridged successfully]
    D -->|202 Processing| F[Retry up to 5x with 2s delay]
    D -->|404 Not Found| G[Immediate hard error]
    
    G --> H[fmt.Errorf returned]
    H --> I[Entire message dropped]
    I --> J[User never sees the photo on Beeper]
    
    F -->|Eventually 200| E
    F -->|Still 202| H
    
    style G fill:#e76f51,color:#fff
    style I fill:#e76f51,color:#fff
    style J fill:#e76f51,color:#fff

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Root cause: LINE's OBS returns transient 404s for recently uploaded media (race condition between upload confirmation and storage propagation). The bridge treated 404 as a permanent failure. When any single download in a bundle failed, the entire message was dropped with no trace on Beeper.

Solution (after)

flowchart TD
    A[Bundle of photos sent on LINE] --> B[Bridge receives multiple messages]
    B --> C[Download media from OBS for each]
    
    C --> D{OBS response?}
    D -->|200 OK| E[Media bridged successfully]
    D -->|202 Processing| F[Retry up to 5x with 2s delay]
    D -->|404 Not Found| F
    
    F -->|Eventually 200| E
    F -->|Still failing| G[Send placeholder message]
    G --> H["[Image unavailable — LINE media expired before it could be bridged]"]
    
    H --> I[User sees the placeholder on Beeper]
    E --> J[User sees the photo on Beeper]
    
    style E fill:#2d6a4f,color:#fff
    style J fill:#2d6a4f,color:#fff
    style I fill:#e9c46a,color:#000

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Two improvements:

1. 404 retry — transient 404s are retried alongside 202s (up to 5x with 2s delay), giving OBS time to propagate
2. Graceful degradation — if download still fails after retries, a placeholder notice is sent instead of silently dropping the message

Test plan

• Send a bundle of photos in a LINE chat and verify all are received on the bridge side
• Verify single image/video/file downloads still work normally

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