Optimize top-payers heap for large heaps

**Description:**

The `get_top_payers` function uses O(n²) selection. For large heaps, this is inefficient.

**Why it matters:** Heap performance affects dashboard load times.

**Acceptance Criteria:**
- [ ] Analyze current implementation
- [ ] Implement more efficient top-k selection
- [ ] Benchmark before/after

**Relevant Files:** `contracts/invoice_liquidity/src/top_payers.rs:163-199`

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