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/009_sessions.sql

@@ -0,0 +1,8 @@
+CREATE TABLE IF NOT EXISTS sessions (
+    token BLOB PRIMARY KEY,
+    username TEXT NOT NULL,
+    identity_key BLOB NOT NULL,
+    created_at INTEGER NOT NULL,
+    expires_at INTEGER NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idx_sessions_expires ON sessions(expires_at);