feat(phase5): scheduling/availability - DB, API, UI in server settings

internal/db/db.go

@@ -435,6 +435,18 @@ CREATE TABLE IF NOT EXISTS list_items (
 
 CREATE INDEX IF NOT EXISTS idx_list_items_channel ON list_items(channel_id, position);
 
+-- Weekly availability
+CREATE TABLE IF NOT EXISTS availability (
+	user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+	server_id UUID NOT NULL REFERENCES servers(id) ON DELETE CASCADE,
+	day_of_week VARCHAR(9) NOT NULL CHECK (day_of_week IN ('monday','tuesday','wednesday','thursday','friday','saturday','sunday')),
+	start_time TIME NOT NULL,
+	end_time TIME NOT NULL,
+	PRIMARY KEY (user_id, server_id, day_of_week, start_time)
+);
+
+CREATE INDEX IF NOT EXISTS idx_availability_server ON availability(server_id, day_of_week);
+
 -- Message full-text search
 ALTER TABLE messages ADD COLUMN IF NOT EXISTS search_vector tsvector;
 CREATE INDEX IF NOT EXISTS idx_messages_search ON messages USING GIN(search_vector);