feat(perf): implement performance optimizations for session handling

- Introduced a new configuration file `config.yaml` for specifying project context and artifact rules.
- Added `.openspec.yaml` files for tracking changes related to performance improvements.
- Created design documents outlining the context, goals, decisions, and migration plans for optimizing session performance.
- Proposed changes include batching database queries, debouncing event refreshes, purging old events, and implementing loading states for better user experience.
- Added tasks and specifications to ensure proper implementation and validation of the new features.

These enhancements aim to improve the scalability and responsiveness of the application during collaborative sessions.

openspec/changes/perf-realtime-scale/specs/unified-broadcast/spec.md

@@ -0,0 +1,22 @@
+## 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