build: add editorconfig, analyzers and CI formatting check

[jonas1307]

What

Introduces code style and linting tooling integrated with the build.

Tooling

• .editorconfig — style rules aligned to the existing conventions (file-scoped namespaces, usings outside the namespace, 4-space indent, var/expression-bodied/collection-expression preferences). EF Core migrations are excluded as generated code.
• Directory.Build.props — enables .NET analyzers + EnforceCodeStyleInBuild for all projects as warnings (conservative profile: surfaces issues without breaking the build).
• .github/workflows/ci.yml — runs on pull requests to master: restore → dotnet format whitespace --verify-no-changes → Release build.

Normalization applied

• UTF-8 BOM removal and final-newline fixes across 12 files (whitespace only).
• Two remaining block-scoped namespaces converted to file-scoped, making the codebase uniform.

Why

There was no formatter or linter wired into the build, so style drift wasn't caught anywhere. This adds a predictable, low-friction gate.

Notes for reviewers

• The CI gate enforces whitespace only (dotnet format whitespace). Full dotnet format was deliberately avoided because its analyzer pass applied unwanted semantic rewrites (e.g. making DbSet<> properties nullable). The analyzers still run during dotnet build as informational warnings.
• Migrations are excluded from formatting to avoid hundreds of lines of churn in generated files.
• end_of_line is intentionally not enforced in .editorconfig and the existing .gitattributes is untouched, to avoid fighting Git's autocrlf on Windows.
• Build is clean: 0 warnings, 0 errors; whitespace verification passes.

🤖 Generated with Claude Code