[photos][iOS] Backup silently and slowly fails when no local storage left on device during backup

[yborunov]

Description

When iOS device storage is full (or too low), Ente can fail during backup while fetching/caching originals from iCloud Photos or writing temporary encrypted upload files.
Today this appears to fail as generic upload/file errors and retries, but without a clear user-facing reason like “Device storage is full”.
This makes backup look stuck/indefinitely retrying and users don’t know they must free local space.

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Steps to reproduce

1. Use Ente Photos on iOS with iCloud Photos enabled.
2. Keep device local storage nearly full.
3. Start/continue backup of iCloud-backed assets.
4. Observe backup progress (looping through assets fast) and no error surfacing other than retry / contact support button.

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Actual behavior

• Individual uploads fail and are retried.
• Backup may remain in retry/error-ish state without clear diagnosis.
• No explicit notification/banner indicating local device storage is full.

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Expected behavior

When backup fails due to local storage exhaustion (e.g. inability to materialize source file/cache or write temp encrypted files), Ente should:

1. Detect and classify this as a local-disk-space error.
2. Surface a clear message in backup status (and optionally push/local notification), e.g.:
   • “Backup paused: device storage is full. Free up space and retry.”
3. Avoid noisy blind retries until storage recovers (or retry with backoff + clear UX state).
4. Optionally provide quick action/link to storage guidance.

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Why this matters

For iOS + iCloud Photos flows, temporary local storage is required for backup. Without explicit handling, users cannot distinguish:

• network issues,
• cloud quota issues,
• local device storage exhaustion.
  This increases support load and causes confusion about backup reliability.

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Platform / product

• Product: Ente Photos
• Platform: iOS
• Area: Backup / Sync