Fix vault data loss when hiding videos

[anilbeesetti]

Summary

• generate unique UUID-backed vault destinations to prevent same-name collisions
• reserve database rows before destructive file operations and reconcile failed or cancelled moves
• copy, sync, and verify each vault file before deleting its source, preserving originals on every failure path
• retain committed batch results while leaving uncommitted sources untouched on cancellation

Reproduction

Issue #1828 was reproduced with two hash-distinct videos sharing the same filename. The old flow overwrote the first vault path and then crashed on the hidden_video.vault_path unique constraint after moving the source.

Testing

• ktlintCheck
• testDebugUnitTest
• app assembleDebug
• 5 core:data instrumentation tests on a disposable Android 16 emulator
• 5 core:media instrumentation tests on a disposable Android 16 emulator
• manual permission-denial and same-name vault QA

Closes #1828