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/src/config.rs

@@ -38,6 +38,8 @@ pub struct FileConfig {
     pub redact_logs: Option<bool>,
     /// WebSocket JSON-RPC bridge listen address (e.g. "0.0.0.0:9000").
     pub ws_listen: Option<String>,
+    /// WebTransport (HTTP/3) listen address for browser clients (e.g. "0.0.0.0:7443").
+    pub webtransport_listen: Option<String>,
     /// Graceful shutdown drain timeout in seconds.
     pub drain_timeout_secs: Option<u64>,
     /// Default per-RPC timeout in seconds.
@@ -70,6 +72,8 @@ pub struct EffectiveConfig {
     pub redact_logs: bool,
     /// WebSocket JSON-RPC bridge listen address. If set, the bridge is started.
     pub ws_listen: Option<String>,
+    /// WebTransport (HTTP/3) listen address. If set, the WebTransport endpoint is started.
+    pub webtransport_listen: Option<String>,
     /// Graceful shutdown drain timeout in seconds.
     pub drain_timeout_secs: u64,
     /// Default per-RPC timeout in seconds.
@@ -250,6 +254,11 @@ pub fn merge_config(args: &crate::Args, file: &FileConfig) -> EffectiveConfig {
         .clone()
         .or_else(|| file.ws_listen.clone());
 
+    let webtransport_listen = args
+        .webtransport_listen
+        .clone()
+        .or_else(|| file.webtransport_listen.clone());
+
     let drain_timeout_secs = if args.drain_timeout == DEFAULT_DRAIN_TIMEOUT_SECS {
         file.drain_timeout_secs.unwrap_or(DEFAULT_DRAIN_TIMEOUT_SECS)
     } else {
@@ -283,6 +292,7 @@ pub fn merge_config(args: &crate::Args, file: &FileConfig) -> EffectiveConfig {
         plugin_dir,
         redact_logs,
         ws_listen,
+        webtransport_listen,
         drain_timeout_secs,
         rpc_timeout_secs,
         storage_timeout_secs,