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 and did not parse as JSON.

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 and loses the trailing comma after logging that kept it from parsing.
• Tests pin that disaster_recovery deserializes, that an sftp replica fails with an unknown variant error, and that a leftover litestream key is ignored.

Breaking changes

• A config that still uses plus.litestream is ignored, so disaster recovery does not run at all: no restore at startup and no replication. Rename the key to plus.disaster_recovery.
• The sftp replica scheme is removed. file and s3 remain. A config that still uses sftp fails to load.

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	5.34 µs (+5.85%) Baseline: 5.04 µs	6.15 µs (86.86%)
Adapter::Magic (JSON)	📈 view plot 🚷 view threshold	5.14 µs (+5.63%) Baseline: 4.87 µs	5.82 µs (88.41%)
Adapter::Magic (Rust)	📈 view plot 🚷 view threshold	26.74 µs (-0.76%) Baseline: 26.94 µs	30.13 µs (88.76%)
Adapter::Rust	📈 view plot 🚷 view threshold	4.68 µs (+8.20%) Baseline: 4.32 µs	6.64 µs (70.41%)
Adapter::RustBench	📈 view plot 🚷 view threshold	4.66 µs (+8.01%) Baseline: 4.32 µs	6.63 µs (70.28%)

🐰 View full continuous benchmarking report in Bencher