rm-hull / fuel-prices-api

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feat: add price statistics to search response (#12)

* feat: add price statistics to search response

Adds a new `statistics` field to the search API response to provide
insights into fuel price data.

- Create `stats` package to calculate averages, price distribution, and
  standard deviation.
- Update `SearchResponse` model to include statistical metadata.
- Enable `--debug` flag in VS Code launch configuration.
- Update test coordinates in `test_api.http`.

```mermaid
sequenceDiagram
    participant C as Client
    participant R as Search Route
    participant S as Stats Package
    C->>R: GET /search
    R->>R: Fetch results from Repository
    R->>S: Derive(results)
    S-->>R: SearchStatistics
    R-->>C: SearchResponse (Results + Statistics)
```

* perf: optimize variance calculation

Replace `math.Pow` with direct multiplication when calculating variance.
Squaring a number via multiplication is more computationally efficient
than calling the power function.

* refactor: consolidate loops in stats derivation

Consolidate statistics calculation logic to reduce redundant
iterations over search results and fuel price arrays.

*   Move **Brand Distribution** counting into the main results loop.
*   Combine **Standard Deviation** variance accumulation with the price
    distribution bucketing logic.

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