WIP: add OPAQUE password-authenticated key exchange

Add opaque-ke (v4, ristretto255) for password-based registration and
login. Extend NodeService schema with opaqueRegisterStart/Finish and
opaqueLoginStart/Finish RPCs. Add Store trait methods for OPAQUE server
setup and user records. Initial e2e integration test scaffolding.

Note: FileBackedStore does not yet implement the new Store trait
methods — server compilation is temporarily broken.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/quicnprotochat-server/Cargo.toml

@@ -32,6 +32,10 @@ quinn-proto   = { workspace = true }
 rustls        = { workspace = true }
 rcgen         = { workspace = true }
 
+# Crypto — OPAQUE PAKE
+opaque-ke     = { workspace = true }
+rand          = { workspace = true }
+
 # Database
 rusqlite      = { workspace = true }