Add optional snapshot compression defaults and standby integration

[sjmiller609]

Summary

• add optional snapshot compression config (zstd/lz4) to snapshot and instance APIs
• add server-level and per-instance snapshot compression defaults with precedence: request override > instance policy > server default
• add async snapshot memory compression jobs for standby and centrally stored standby snapshots
• add restore-time snapshot memory preparation (cancel/wait active compression + on-demand decompression)
• include compression metadata/state on snapshot resources

API / Config

• POST /instances/{id}/standby accepts optional compression
• POST /instances/{id}/snapshots accepts optional compression
• POST /instances accepts optional snapshot_policy
• config adds snapshot.compression_default (enabled, algorithm, level)

Validation and behavior

• compression is standby-only (rejected for stopped snapshots)
• zstd level validated (1-19)
• lz4 rejects level input
• standby validates compression policy before state transitions

Tests

• go test -run '^$' ./cmd/api/api ./lib/instances ./lib/providers ./integration
• go test ./lib/instances -run 'TestNormalizeCompressionConfig|TestResolveSnapshotCompressionPolicyPrecedence|TestValidateCreateRequestSnapshotPolicy|TestValidateCreateSnapshotRequestRejectsStoppedCompression'