Make plus.disaster_recovery the only key and drop sftp

[epompeii]

The continuous replication section of the server config was named plus.litestream in code, with disaster_recovery accepted only as a serde alias, while the docs have always named it plus.disaster_recovery. This makes the documented key the only key, drops the unused sftp replica scheme, and fixes the docs example, which showed a replicas array the code never accepted.

Changes

• JsonPlus.litestream becomes disaster_recovery: Option<JsonDisasterRecovery>; the alias is gone. The code that runs the Litestream binary keeps its Litestream names, since that is the vendor it drives.
• JsonReplica::Sftp and its YAML mapping are removed, so the section reads scheme: file | s3.
• Docs in all 9 languages drop the sftp scheme; the shared config example shows a single replica.
• Tests pin that disaster_recovery deserializes for file and s3, that sftp is rejected, and that a leftover litestream key is ignored.

Breaking changes

• A config that still uses plus.litestream is ignored and continuous replication does not start. Rename the key to plus.disaster_recovery.
• The sftp replica scheme is removed. file and s3 remain.

First of a stack of three: this, then removing the on demand backup endpoint, then moving the container registry storage onto object_store with the same scheme: file | s3 config shape.

[github-actions]

Bencher Report

Project	Bencher
Branch	u/ep/s3-r2/disaster-recovery
Testbed	intel-v1

Click to view all benchmark results

Benchmark	Latency	Benchmark Result microseconds (µs) (Result Δ%)	Upper Boundary microseconds (µs) (Limit %)
Adapter::Json	📈 view plot 🚷 view threshold	5.18 µs (+7.21%) Baseline: 4.83 µs	5.84 µs (88.70%)
Adapter::Magic (JSON)	📈 view plot 🚷 view threshold	4.96 µs (+5.94%) Baseline: 4.68 µs	5.56 µs (89.16%)
Adapter::Magic (Rust)	📈 view plot 🚷 view threshold	27.50 µs (+4.38%) Baseline: 26.34 µs	29.62 µs (92.83%)
Adapter::Rust	📈 view plot 🚷 view threshold	4.68 µs (+22.02%) Baseline: 3.84 µs	5.90 µs (79.38%)
Adapter::RustBench	📈 view plot 🚷 view threshold	4.66 µs (+21.65%) Baseline: 3.83 µs	5.88 µs (79.31%)

🐰 View full continuous benchmarking report in Bencher