feat: NIP-AB device pairing — Phase 1 (core library + CLI) (#333)

schema/schema.sql

@@ -41,7 +41,8 @@ CREATE TABLE channels (
     purpose_set_at  TIMESTAMPTZ,
     participant_hash BYTEA,
     ttl_seconds     INT,
-    ttl_deadline    TIMESTAMPTZ
+    ttl_deadline    TIMESTAMPTZ,
+    CONSTRAINT chk_channels_id_not_nil CHECK (id <> '00000000-0000-0000-0000-000000000000'::uuid)
 );
 
 CREATE INDEX idx_channels_type ON channels (channel_type);