[A-EDev]

Summary

On a phone with a large library, every linked sync exported the whole library to compare it with what was already sent, and a merge first wrote a full-profile backup. With another device playing that ran every 12 to 45 s; logcat showed 40 MB and 96 to 120 MB large-object bursts at each sync and 3 to 6 s backups, with the heap reaching 245 of 256 MB.

• The pre-merge backup is taken only when a merge touches likes, settings, subscribed channels or a brain. Watch history, subscription groups, notes and playlists are written inside one Room transaction, which already undoes a failed merge.
• A push exports only what can have changed: a brain once its 6-hour upload wait is over (it was exported every sync and then held back), a Room collection once Room's InvalidationTracker saw its tables change or while the ledger holds nothing of it (after a reset), every other collection as before. Skipped collections would have sent nothing.
• The brain export writes straight into its stream instead of building a string and its bytes first.

Related issue

Refs #1412

Change type

• Bug fix
• Feature
• Refactor or maintenance
• Build, packaging, or CI
• Documentation

Validation

• ./gradlew :app:assembleGithubDebug: passed
• ./gradlew :app:testGithubDebugUnitTest: 4717 tests, 0 failures (2 skipped), including the new PushPlanTest and the new ApplyOnceTest case
• I ran any additional flavor-specific build or test tasks affected by this change: :app:compileFossDebugKotlin passed, ktlintCheck passed, python3 .github/scripts/nightly_notes.py --check passed
• I manually tested the affected behavior on an Android device or emulator.

Test device and Android version:

Screenshots or recordings

Risk and compatibility

• No Room schema change: one new read query on the linked-devices sent table.

• What is uploaded and merged is unchanged; only when exports and backups run changes. A failed push marks its Room collections as changed again so the next push exports them.

• Battery and memory: fewer exports and backups per sync. Not yet measured on a device.

• The change does not introduce secrets, private data, or unexpected telemetry.

• New user-facing text uses Android string resources.

• Dependency and lockfile changes are intentional and limited to this PR.

• Room schema changes include the required version bump and migration, or this PR does not change the Room schema.

• Breaking changes and upgrade steps are clearly documented.