remove sqlx MySQL support to eliminate RSA transitive vulnerability

[bug-ops]

Summary

• Disable sqlx default features in workspace Cargo.toml, explicitly listing only needed features (macros)
• Per-crate sqlx dependencies already specify runtime-tokio, sqlite, migrate as needed
• Eliminates sqlx-mysql → rsa transitive dependency (RUSTSEC-2023-0071)
• cargo deny check passes clean; cargo audit still flags the lockfile entry (known limitation — optional deps remain in Cargo.lock)

Test plan

• cargo build --workspace succeeds
• cargo nextest run --workspace --lib --bins: 1351 tests pass
• cargo deny check advisories: no vulnerabilities
• cargo tree confirms sqlx-mysql is not in resolved dependency graph

Closes #305

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
+ Coverage   79.96%   80.01%   +0.05%     
==========================================
  Files          98       98              
  Lines       23964    23926      -38     
==========================================
- Hits        19163    19145      -18     
+ Misses       4801     4781      -20     

see 3 files with indirect coverage changes

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