fixed: variant count endpoint was returning a count including non-distinct variants

[bencap]

This pull request refactors the variant counting logic in the src/mavedb/routers/statistics.py file to improve performance and fix the count of variants. The main changes are explicitly querying against distinct variant identifiers and the introduction of a fast path for total counts, which avoids unnecessary grouping and aggregation when not requested, and a clearer separation between grouped and ungrouped queries.

Performance and Query Logic Improvements:

• Explicitly query against distinct variant identifiers, so as to avoid double counting variants when they are materialized as more than one row. [1] [2]
• Added a fast path to both variant_counts and mapped_variant_counts functions to quickly return the total distinct count when no grouping is requested, reducing query complexity and improving performance. [1] [2]
• Refactored grouped queries to first materialize per-date distinct counts, then aggregate by month or year as needed, ensuring correct results and simplifying the code. [1] [2]
• Added defensive fallback logic to ensure that even in unexpected cases, the functions return a valid count. [1] [2]