entities: enforce immutability doctrine across domain models#125
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Domain models are records — snapshots of business state — not stateful objects. Without an explicit contract, mutable collections creep in (list, dict), direct field assignment happens in use cases and repositories, and the distinction between "changing an entity" and "creating a new snapshot" becomes unclear. This matters especially in Temporal workflows where shared in-memory state across signal handlers and activity calls is a source of subtle replay bugs. A new Entity base class in core/entities/entity.py establishes the contract: frozen=True prevents field reassignment, and doctrine tests enforce that all domain models inherit from Entity and use only immutable collection annotations (tuple, Mapping, frozenset). All ceap and polling domain models now extend Entity. Mutable list/dict annotations have been replaced with tuple/Mapping. The repository base classes (memory and minio) previously mutated entity fields directly in update_timestamps — these now return new instances via model_copy. Use cases that assigned to entity fields mid-workflow do the same.
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Replacement for auto-closed PR #120.