## ADDED Requirements

### Requirement: Centralized broadcast module
The system SHALL provide a centralized `src/lib/broadcast.ts` module used by all workshop SSE routes to push events to connected clients.

#### Scenario: Server Action triggers broadcast
- **WHEN** a Server Action mutates session data and calls `broadcast(sessionId, event)`
- **THEN** all clients subscribed to that session SHALL receive the event immediately without waiting for the next poll cycle

#### Scenario: Broadcast module subscribe/unsubscribe
- **WHEN** an SSE route calls `subscribe(sessionId, callback)`
- **THEN** the callback SHALL be invoked on every subsequent `broadcast(sessionId, ...)` call
- **WHEN** the returned `unsubscribe()` function is called
- **THEN** the callback SHALL no longer receive events

### Requirement: Granular cache invalidation via revalidateTag
Server Actions SHALL use `revalidateTag` with session-scoped tags instead of `revalidatePath` to limit cache invalidation scope.

#### Scenario: Session mutation invalidates only that session's cache
- **WHEN** a Server Action mutates a specific session (e.g., adds an item)
- **THEN** only the cache tagged `session:<id>` SHALL be invalidated
- **THEN** other sessions' cached data SHALL NOT be invalidated