Add embedded web chat client (closes #7) (#8)

internal/db/schema/003_users.sql

@@ -0,0 +1,31 @@
+PRAGMA foreign_keys = ON;
+
+CREATE TABLE IF NOT EXISTS users (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    nick TEXT NOT NULL UNIQUE,
+    token TEXT NOT NULL UNIQUE,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    last_seen DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS channel_members (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    channel_id INTEGER NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    joined_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE(channel_id, user_id)
+);
+
+CREATE TABLE IF NOT EXISTS messages (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    channel_id INTEGER REFERENCES channels(id) ON DELETE CASCADE,
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    content TEXT NOT NULL,
+    is_dm INTEGER NOT NULL DEFAULT 0,
+    dm_target_id INTEGER REFERENCES users(id) ON DELETE CASCADE,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX IF NOT EXISTS idx_messages_channel ON messages(channel_id, created_at);
+CREATE INDEX IF NOT EXISTS idx_messages_dm ON messages(user_id, dm_target_id, created_at);
+CREATE INDEX IF NOT EXISTS idx_users_token ON users(token);