The active flag was synthesized as always-true. Now it's a real top-level field
on the Medication document (serde default=true so old rows stay active),
wired through create (default true), update ($set the field, not the data blob),
and list (the previously-ignored ?active= query param now filters at Mongo level
via find_by_user_filtered). Fixed stale comments claiming updates.active was
'accepted but not persisted' (the field didn't even exist on UpdateMedicationRequest).
Verified: cargo fmt/clippy 0 warnings, 23 tests pass; frontend build + 20 tests.
0921d73a6d