Add conventions: local functions, tests vs production code, EF navigations

[KaliCZ]

Three written conventions (wording taken from the global rules), all docs-only — the codebase already complies with each. Each lives in CLAUDE.md and docs/technical-requirements.md.

No nested local functions — write a normal method

Don''t reach for local functions; extract a private method instead. A one-liner local function can occasionally be acceptable, but that''s an edge case, not something to aim for. No Roslyn analyzer covers this, so it lives as a written convention.

Verified: swept all C# for local-function declaration shapes (modifier-prefixed, built-in/custom/tuple return types, expression-bodied, top-level-statement files) — zero hits.

Tests never reshape production code

We use DDD: the domain model is designed for the domain, not for testability. If a test needs some capability, the test figures out how to achieve it — never add methods, parameters, setters, or hooks to production code just so a test can reach something. Dependency injection should be enough; otherwise the test manipulates state itself (seeding through EF, reflection as a last resort).

Verified: all entity properties are private set, no InternalsVisibleTo anywhere, tests seed exclusively through domain entities + EF. (public partial class Program; stays — the standard WebApplicationFactory host idiom, not a domain signature.)

Entity Framework: set the navigation property together with the FK

When assigning a relationship by ID (e.g. ProductId), also assign the navigation property (Product) — in constructors, factory methods, and setters alike — so that right after the call, something.Product is usable and non-null. The exception is performance-sensitive work like inserting thousands of rows in a batch: there it''s fine to set only the ID rather than load entities into memory that nothing will read.

Verified: the current model has no reference navigations at all (Product.Reviews and Review.Votes are collection navigations on the principal side; dependents carry only FKs), so no call site can violate the rule today — it binds the moment a reference navigation is introduced.

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