Concurrent Per-Thread Generations (M2 #21)

[ddroid]

What to build

Upgrade from M0's single-global-generation model to support concurrent generations across different threads. Each thread should be able to have its own active generation independently. Users can switch between threads and send messages in different threads without waiting for other threads' generations to complete.

Cancel should apply to the generation in the currently viewed thread.

Acceptance criteria

• User can start a generation in Thread A, switch to Thread B, and start another generation
• Both generations stream independently without interfering
• Each thread shows its own streaming/complete/failed/cancelled status independently
• Cancel button cancels the generation in the currently viewed thread only
• Thread list in sidebar shows which threads have active generations
• Switching to a thread with an active generation shows the live stream
• Multiple concurrent generations do not cause database contention or corruption
• Tests cover concurrent generation lifecycle through public interfaces

Blocked by

None - can start immediately