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/003_channels.sql

@@ -0,0 +1,13 @@
+-- Migration 003: 1:1 DM channels.
+-- channel_id is 16 bytes (UUID); member_a and member_b are identity keys in sorted order.
+-- Unique on (member_a, member_b) prevents duplicate channels between the same pair.
+
+CREATE TABLE IF NOT EXISTS channels (
+    channel_id BLOB PRIMARY KEY,
+    member_a   BLOB NOT NULL,
+    member_b   BLOB NOT NULL,
+    UNIQUE(member_a, member_b)
+);
+
+CREATE INDEX IF NOT EXISTS idx_channels_members
+    ON channels(member_a, member_b);