Gate Diesel backends with Cargo features

[leynos]

Summary

• disable default Diesel features and add sqlite/postgres features
• enable Diesel and diesel-async backend support via Cargo features
• require feature selection when building
• silence markdownlint line-length warnings

Testing

• cargo clippy --features sqlite -- -D warnings
• cargo test --features sqlite (fails: ReturningClause not supported for Sqlite)
• markdownlint README.md docs/supporting-both-sqlite3-and-postgresql-in-diesel.md
• nixie docs/supporting-both-sqlite3-and-postgresql-in-diesel.md README.md

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https://chatgpt.com/codex/tasks/task_e_6848cc3408d4832294d6f7e4ae78005b

Summary by Sourcery

Gate Diesel and Diesel-async backends behind explicit Cargo features, disable their default features, and update documentation to require selecting either the sqlite or postgres feature when building.

New Features:

• Add sqlite and postgres Cargo features to enable corresponding Diesel and Diesel-async backends

Enhancements:

• Disable default features for diesel, diesel-async, and libsqlite3-sys dependencies
• Add bundled libsqlite3-sys dependency for sqlite support

Build:

• Require explicit feature selection for database backends in Cargo.toml

Documentation:

• Update README and supporting documentation to instruct building with --features sqlite or --features postgres
• Silence markdownlint line-length warnings in documentation

[codescene-delta-analysis]

Gates Passed
6 Quality Gates Passed

See analysis details in CodeScene

Quality Gate Profile: Pay Down Tech Debt
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[sourcery-ai]

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