feat(platform/copilot): live timer stats with persisted duration (#12583)

## Why

The copilot chat had no indication of how long the AI spent "thinking"
on a response. Users couldn't tell if a long wait was normal or
something was stuck. Additionally, the thinking duration was lost on
page reload since it was only tracked client-side.

## What

- **Live elapsed timer**: Shows elapsed time ("23s", "1m 5s") in the
ThinkingIndicator while the AI is processing (appears after 20s to avoid
spam on quick responses)
- **Frozen "Thought for Xm Ys"**: Displays the final thinking duration
in TurnStatsBar after the response completes
- **Persisted duration**: Saves `durationMs` on the last assistant
message in the DB so the timer survives page reloads

## How

**Backend:**
- Added `durationMs Int?` column to `ChatMessage` (Prisma migration)
- `mark_session_completed` in `stream_registry.py` computes wall-clock
duration from Redis session `created_at` and saves it via
`DatabaseManager.set_turn_duration()`
- Invalidates Redis session cache after writing so GET returns fresh
data

**Frontend:**
- `useElapsedTimer` hook tracks client-side elapsed seconds during
streaming
- `ThinkingIndicator` shows only the elapsed time (no phrases) after
20s, with `font-mono text-sm` styling
- `TurnStatsBar` displays "Thought for Xs" after completion, preferring
live `elapsedSeconds` and falling back to persisted `durationMs`
- `convertChatSessionToUiMessages` extracts `duration_ms` from
historical messages into a `Map<string, number>` threaded through to
`ChatMessagesContainer`

## Test plan

- [ ] Send a message in copilot — verify ThinkingIndicator shows elapsed
time after 20s
- [ ] After response completes — verify "Thought for Xs" appears below
the response
- [ ] Refresh the page — verify "Thought for Xs" still appears
(persisted from DB)
- [ ] Check older conversations — they should NOT show timer (no
historical data)
- [ ] Verify no Zod/SSE validation errors in browser console

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>