Serialize dataclasses & rich types in mk_app responses

[thorwhalen]

Summary

qh.mk_app() endpoints returned HTTP 500 when the wrapped function
returned a value the stdlib json encoder can't handle directly — a
@dataclass, a pydantic model, datetime, Enum, set, UUID,
Decimal, etc.:

Error in <func>: Object of type <T> is not JSON serializable

endpoint.py built the success response with JSONResponse(content=output);
Starlette's JSONResponse.render() calls json.dumps with the default
encoder. A TypedDict instance is a plain dict at runtime so it worked — a
dataclass instance is not, so it raised and qh's generic except wrapped it
as a 500.

Fix

Run the output through FastAPI's jsonable_encoder before constructing the
JSONResponse. It recursively converts dataclasses, pydantic models (v1/v2),
datetime, Enum, set, UUID, Decimal, … into JSON-compatible
primitives. Plain dict/list/scalar outputs pass through unchanged, so the
change is non-breaking — it strictly broadens what serializes.

Tests

Three new tests in test_mk_app.py — a dataclass return, a list of
dataclasses (the ef.EfService.search() shape that surfaced this), and
datetime / Enum. Full suite: 139 passed.

Closes #6