Each root namespace (Audit, Common, Data, Environments, Features/Usage,
Identities, Organizations, Projects, Segments, Users, Webhooks) gets its
own static DependencyRegistration class exposing an Add<Name>Services
extension method, so Program.cs just wires them together instead of
listing every registration inline.
Consumers now depend on IAuditService instead of the concrete class,
so tests mock the interface (Mock<IAuditService>()) rather than
subclassing AuditService via virtual methods.
WebhookEvent, FeatureStateResult, ProjectPermissionRequirement, AuditLog,
Tag, and FeatureUsageDaily are plain data holders with no behavior, so
switch them to records. EF-mapped entities with identity semantics
(User, Organization, Project, Feature, Segment, etc.) stay classes,
since record value-equality and deep ToString fight EF's change tracker.
Also restore `virtual` on AuditService.RecordAsync/LogAsync, which had
been dropped and broke Mock<AuditService>-based test setups.
Groups usage-tracking (metrics, background flush, query service,
controller, entity) under its own namespace/folder, updates all
consumers and the EF migration snapshot's CLR type strings to match.
Replace AbstractValidator-based request validators with a small
IModelValidator<T>/ValidationResult pattern (ported from BuyEngine's
Common namespace), wired into the MVC pipeline via a
ModelValidationActionFilter so controllers and response shape are
unaffected. Also copy over the Guard precondition helper.
