feat: initialize Discord clone application with core backend services and Electron frontend.

Backend/schema.sql

@@ -16,7 +16,7 @@ CREATE TABLE IF NOT EXISTS users (
 CREATE TABLE IF NOT EXISTS channels (
     id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
     name TEXT UNIQUE NOT NULL,
-    type TEXT DEFAULT 'text' CHECK (type IN ('text', 'voice')),
+    type TEXT DEFAULT 'text' CHECK (type IN ('text', 'voice', 'dm')),
     created_at TIMESTAMPTZ DEFAULT NOW()
 );
 
@@ -65,3 +65,9 @@ CREATE TABLE IF NOT EXISTS invites (
     key_version INTEGER NOT NULL,        -- Which key version is inside? (So we can invalidate leaks)
     created_at TIMESTAMPTZ DEFAULT NOW()
 );
+
+CREATE TABLE IF NOT EXISTS dm_participants (
+    channel_id UUID REFERENCES channels(id) ON DELETE CASCADE,
+    user_id UUID REFERENCES users(id) ON DELETE CASCADE,
+    PRIMARY KEY (channel_id, user_id)
+);