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/sessions-list/spec.md

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+## ADDED Requirements
+
+### Requirement: Paginated sessions list
+The sessions page SHALL load sessions in pages rather than fetching all sessions at once, with a default page size of 20 per workshop type.
+
+#### Scenario: Initial load shows first page
+- **WHEN** a user visits the sessions page
+- **THEN** at most 20 sessions per workshop type SHALL be loaded
+- **THEN** a total count SHALL be displayed (e.g., "Showing 20 of 47")
+
+#### Scenario: Load more sessions on demand
+- **WHEN** there are more sessions beyond the current page
+- **THEN** a "Charger plus" button SHALL be displayed
+- **WHEN** the user clicks "Charger plus"
+- **THEN** the next page of sessions SHALL be appended to the list

openspec/changes/perf-realtime-scale/specs/sse-shared-polling/spec.md

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+## ADDED Requirements
+
+### Requirement: Single polling interval per active session
+The SSE infrastructure SHALL maintain at most one active DB polling interval per session, regardless of the number of connected clients.
+
+#### Scenario: First client connects starts polling
+- **WHEN** the first client connects to a session's SSE endpoint
+- **THEN** a single polling interval SHALL be started for that session
+
+#### Scenario: Additional clients share existing polling
+- **WHEN** a second or subsequent client connects to the same session's SSE endpoint
+- **THEN** no additional polling interval SHALL be created
+- **THEN** the new client SHALL receive events from the shared poll
+
+#### Scenario: Last client disconnect stops polling
+- **WHEN** all clients disconnect from a session's SSE endpoint
+- **THEN** the polling interval for that session SHALL be stopped and cleaned up

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

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+## 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