Remove periodic data transfer

[CGodiksen]

This PR removes periodic data transfer due to reliability and consistency issues. Event-driven data transfer based on the batch size is still available.

The periodic data transfer is removed since it was the only part of the system where we had a time-based background task (or spawned background task in general) instead of event-driven handling. This change makes data transfer consistent with the rest of the system.

Furthermore, reliability issues was found due to potential concurrency problems. A background task running data transfer could overlap with optimize/vacuum/truncate and cause issues. This becomes an even larger problem when an automatic (event-driven) data optimizer is added.

When reviewing the PR, also consider whether these reasons are enough to justify the removal.

[Copilot]

Pull request overview

This PR removes the time-based (periodic) data transfer mechanism from the server, leaving only event-driven transfer based on batch size (plus explicit transfer actions), in order to improve reliability and avoid overlapping background work with other storage operations.

Changes:

• Remove transfer_time_in_seconds from CLI/configuration, server configuration management, and remote UpdateConfiguration handling.
• Remove the periodic transfer scheduler task from the data transfer component and storage engine initialization.
• Update documentation/tests to reflect the removed setting.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
docs/user/README.md	Removes the documented --transfer-time-in-seconds setting from the configuration table.
crates/modelardb_types/src/flight/protocol.proto	Removes transfer_time_in_seconds from the public Flight protobuf surface (but currently renumbers fields/enum values).
crates/modelardb_server/tests/integration_test.rs	Removes assertions/tests related to the removed transfer-time setting.
crates/modelardb_server/src/storage/mod.rs	Stops starting/maintaining the periodic transfer scheduler via the storage engine.
crates/modelardb_server/src/storage/data_transfer.rs	Removes the scheduler handle and periodic transfer logic from DataTransfer.
crates/modelardb_server/src/remote/mod.rs	Removes UpdateConfiguration handling for the removed transfer-time setting.
crates/modelardb_server/src/main.rs	Removes the transfer-time CLI argument and updates the batch-size argument docs.
crates/modelardb_server/src/configuration.rs	Removes transfer-time from persisted configuration and live update logic; updates batch-size docs.

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