Airdrop: add pagination to leaderboard

[realproject7]

Problem

The leaderboard API hardcodes `sorted.slice(0, 50)` and the component shows all entries at once with no pagination. As participants grow, this won't scale.

Solution

API: Add `page` and `limit` query params

File: `src/app/api/airdrop/leaderboard/route.ts`

```
GET /api/airdrop/leaderboard?page=1&limit=20&address=0x...
```

• `page`: 1-indexed page number (default: 1)
• `limit`: entries per page (default: 20, max: 50)
• Keep `address` param for user rank lookup

Response adds pagination metadata:
```json
{
"entries": [...],
"userRank": 67,
"totalParticipants": 150,
"page": 1,
"totalPages": 8,
"limit": 20
}
```

Note: the API currently fetches ALL `pl_points` rows and aggregates in-memory. Pagination only affects the returned slice — the full sort still happens server-side. This is fine for now but may need a materialized view or DB-level aggregation at scale.

Component: Prev/Next pagination

File: `src/components/airdrop/Leaderboard.tsx`

```
┌────────────────────────────────┐
│ Leaderboard │
│ 150 participants │
│ │
│ # User PL Share │
│ 1 alice.eth 1,247 12.5% │
│ 2 0xAB...CD 980 9.8% │
│ ... │
│ 20 0xEF...12 120 1.2% │
│ │
│ ← Prev 1/8 Next → │
│ │
│ Your rank: #67 │
└────────────────────────────────┘
```

• Default 20 entries per page
• Prev/Next buttons with current page / total pages
• User rank always shown at bottom (regardless of page)
• React Query key includes page number for caching

Acceptance Criteria

• API accepts `page` and `limit` query params
• API returns `totalParticipants`, `page`, `totalPages`
• Component shows Prev/Next pagination controls
• Current page / total pages displayed
• User rank still shown when outside current page
• Default: 20 per page