feat: add WebTransport (HTTP/3) server endpoint for browser clients

Feature-gated behind --features webtransport. Uses h3, h3-quinn,
and h3-webtransport crates to accept WebTransport sessions over
HTTP/3. Dispatches RPC through the same v2 handler registry as
native QUIC, using identical wire framing.

- webtransport.rs: H3 connection handling, session management,
  bidi stream RPC dispatch with auth handshake
- Config: --webtransport-listen / QPQ_WEBTRANSPORT_LISTEN
- ALPN: "h3" for WebTransport, "capnp" for native QUIC
- Also fixes: add missing save/load_revocation_log to SqlStore

crates/quicproquo-server/Cargo.toml

@@ -79,7 +79,7 @@ mdns-sd = "0.12"
 
 # WebTransport (HTTP/3) — feature-gated, for browser clients.
 h3                = { version = "0.0.8", optional = true }
-h3-quinn          = { version = "0.0.10", optional = true }
+h3-quinn          = { version = "0.0.10", features = ["datagram"], optional = true }
 h3-webtransport   = { version = "0.1", optional = true }
 http              = { version = "1", optional = true }