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/002_add_seq.sql

@@ -0,0 +1,21 @@
+-- Migration 002: add per-inbox delivery sequence numbers.
+--
+-- Adds a `seq` column to the deliveries table and a separate counter table
+-- that tracks the next sequence number per (recipient_key, channel_id) inbox.
+-- The counter is atomically incremented on each enqueue via an UPSERT so
+-- sequence numbers are gapless even under concurrent inserts.
+--
+-- Requires SQLite >= 3.35 (RETURNING clause support; available on Ubuntu 22.04+).
+
+ALTER TABLE deliveries ADD COLUMN seq INTEGER NOT NULL DEFAULT 0;
+
+CREATE TABLE IF NOT EXISTS delivery_seq_counters (
+    recipient_key BLOB    NOT NULL,
+    channel_id    BLOB    NOT NULL,
+    next_seq      INTEGER NOT NULL DEFAULT 0,
+    PRIMARY KEY (recipient_key, channel_id)
+);
+
+-- Index lets ORDER BY seq queries use an index scan instead of a sort.
+CREATE INDEX IF NOT EXISTS idx_del_seq
+    ON deliveries (recipient_key, channel_id, seq);