chore: rename quicproquo → quicprochat in Rust workspace

Rename all crate directories, package names, binary names, proto
package/module paths, ALPN strings, env var prefixes, config filenames,
mDNS service names, and plugin ABI symbols from quicproquo/qpq to
quicprochat/qpc.

crates/quicprochat-server/migrations/005_signing_key.sql

@@ -0,0 +1,4 @@
+CREATE TABLE IF NOT EXISTS server_signing_key (
+    id        INTEGER PRIMARY KEY CHECK (id = 1),
+    seed_data BLOB NOT NULL
+);