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/007_add_expiry.sql

@@ -0,0 +1,2 @@
+ALTER TABLE deliveries ADD COLUMN expires_at INTEGER;
+CREATE INDEX idx_deliveries_expires ON deliveries(expires_at) WHERE expires_at IS NOT NULL;