feat: Add average order completion time and completion rate metrics to public order listings

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## Summary

Add two per-user metrics: the average time to complete an order (from taken to `SUCCESS`) and the completion rate (percentage of taken orders that reached `SUCCESS`). Both are stored on the User model and displayed in the public channel alongside the order.

## Motivation

When two sellers (or buyers) offer identical terms, there is no way to distinguish them by reliability or speed. A visible completion time and completion rate give users meaningful signals to pick the better counterparty, improving overall trade quality and velocity on the platform.

## Proposed UX

Order listing in the channel would include two new lines:

```
⚡ Sell 100,000 sats
💵 1,000 USD — Bank transfer
👤 @alice ⏱ avg. completion: 12 min ✅ 89% completion rate
```

If the user has fewer than 5 completed orders:

```
👤 @bob ⏱ new trader
```

## Data Model Changes

New fields added to the User model:

- `avg_completion_time` — average time in seconds from order taken to `SUCCESS`
- `total_taken_orders` — total number of orders the user has taken, used as the completion rate denominator

The existing `trades_completed` field is reused as the completion rate numerator.

## Calculation Logic

**Completion rate** — recalculated on every `SUCCESS` transition:

```
completionRate = (trades_completed / total_taken_orders) * 100
```

**Average completion time** — updated on every `SUCCESS` transition using a running average, where elapsed time is measured from `order.taken_at` to the moment the order completes.

## Display Logic

- `trades_completed >= 5` → show both metrics alongside the order in the channel
- `trades_completed < 5` → show `new trader` or omit both metrics entirely

## Localization

New keys needed in all `locales/` files:

- `avg_completion_time` — e.g. "⏱ avg. completion: {{time}} min"
- `completion_rate` — e.g. "✅ {{rate}}% completion rate"
- `new_trader` — e.g. "⏱ new trader"

Start by locating the User model, the order transition to SUCCESS, and the public channel order-listing renderer. Review every file under locales/ for the required keys. Done means completion metrics are stored and updated correctly, the new-trader fallback and metric display follow the five-order threshold, and all locales contain the new strings.

Written by the indexing model from the issue text.