libsql-server: enforce namespace authorization on gRPC replication RPCs

[bartzon]

Problem

The gRPC replication path in libsql-server validated the JWT signature but never checked the token's namespace claim against the namespace being served:

• rpc/replication/auth.rs called auth.authenticate(context)? and discarded the returned Authenticated.
• replication_log.rs (Hello/LogEntries/BatchLogEntries/Snapshot) then called self.namespaces.with(...) directly, so nothing compared p.ro.ns against the namespace being replicated.

Consequence: any valid replication token — including one a legitimate client holds for its own namespace — could replicate any namespace by connecting to that namespace's host. This is the cross-namespace authorization gap behind shop/issues-retail#34480.

Fix

In rpc/replication/auth.rs, use the returned Authenticated and call is_namespace_authorized(&namespace), returning permission_denied on mismatch. All four replication RPCs flow through authenticate(), so this covers the whole path.

is_namespace_authorized on Legacy{ namespace: None } returns true, so legacy/no-namespace tokens (e.g. the CloudSync streamer) still replicate every namespace — no regression for existing server-side callers.

Test

Adds replicate_cross_namespace_denied in tests/embedded_replica/mod.rs:

• ns1-scoped token replicates ns1 (control)
• ns1-scoped token replicating ns2 is denied (the bug)
• token scoped to both namespaces replicates both (control)

Upstream's replicate_auth only asserts token-present vs token-absent; this is the missing negative cross-namespace test.

Verification

• cargo check -p libsql-server and --tests pass clean.
• The new test run requires Linux/CI (libsql-server tests don't link on macOS/arm64) — verified compile-only locally.

[bartzon]

Closing in favor of router-layer enforcement. Per team direction (Rafał), we want to avoid further forking libsql — the namespace-claim check will live in retail-libsql-router (Shopify/libsql-infrastructure), which sees all public and internal traffic anyway. This branch is kept as a reference for the negative cross-namespace test shape.