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/004_federation.sql

@@ -0,0 +1,16 @@
+-- 004_federation.sql: Federation support tables.
+
+-- Map identity keys to their home server domain.
+-- Used for routing: if a recipient's home_server != local domain, relay via federation.
+CREATE TABLE IF NOT EXISTS identity_home_servers (
+    identity_key BLOB PRIMARY KEY,
+    home_server  TEXT NOT NULL,
+    updated_at   INTEGER NOT NULL
+);
+
+-- Known federation peers (other quicnprotochat servers).
+CREATE TABLE IF NOT EXISTS federation_peers (
+    domain    TEXT PRIMARY KEY,
+    last_seen INTEGER NOT NULL,
+    is_active INTEGER NOT NULL DEFAULT 1
+);