Feature: Multi-Workspace Support with Data Isolation and Quick Switching

## Problem
The tool currently assumes a single logical workspace/data context. Users monitoring multiple repositories/teams need separation and quick switching.

## Proposal
Add **Workspace Profiles** with strict data isolation and fast context switching.

## Scope
- Workspace entity with name, root path patterns, color/icon
- Data partitioning per workspace (sessions, agents, events, analytics)
- Workspace switcher in sidebar/header
- Cross-workspace aggregate dashboard (optional)

## UX Requirements
- Last selected workspace persisted across restarts
- Clear visual indicator of active workspace
- Safe fallback if workspace is missing/renamed

## Technical Notes
- Add `workspace_id` to key tables with indexed access paths
- Migration strategy for existing single-workspace data
- Optional workspace filters on all API endpoints

## Acceptance Criteria
- [ ] User can create/edit/archive workspaces
- [ ] New events are attributed to the correct workspace
- [ ] Switching workspace updates all views consistently
- [ ] Existing data migrates to a default workspace automatically
- [ ] API supports both scoped and aggregate queries