fix(medications): serialise unknown inventory as null, not 0

An inventory item's unit count comes from a non-null Decimal column, but a
corrupt or legacy row could hold a missing or non-finite value. The
serialiser ran `Number(item.unitsTotal)`, and `Number(null) === 0` (and
`Number("corrupt") === NaN`), so an unknown count went out on the wire as a
fabricated 0 that a client could decrement into negatives.

Serialise a genuinely-unknown count (null / non-finite) as `null` so clients
render "unknown" instead. A real tracked 0 stays 0. No model change — the
distinction is serialisation-only.

The supply tab now renders an unknown-units row as "—", pre-fills the
correction editor empty, drops the capacity ceiling when total is unknown,
and keeps such a row out of the pooled available-supply headline. The
low-stock alert already excludes non-usable rows, so it does not fire on
unknown stock. OpenAPI marks both unit fields nullable.

docs/api/openapi.yaml

@@ -11972,14 +11972,21 @@ components:
           description: Kind of physical container (PEN / AMPOULE / BLISTER / INHALER / BOTTLE / OTHER). Display-level
             classification; defaults to OTHER.
         unitsTotal:
-          type: number
+          anyOf:
+            - type: number
+            - type: "null"
           description: Units the container shipped with (tablets / ampoules / puffs; 1–1000). v1.16.12 — fractional, so a
-            split-pill remainder reads cleanly. Dose-derived readouts divide by the medication's `unitsPerDose`.
+            split-pill remainder reads cleanly. Dose-derived readouts divide by the medication's `unitsPerDose`. v1.18.3
+            (iOS#31) — NULL when the unit count is unknown (a corrupt or legacy row); the client renders "unknown",
+            never a fabricated 0.
         unitsRemaining:
-          type: number
+          anyOf:
+            - type: number
+            - type: "null"
           description: Units left in the container. v1.16.12 — fractional (a ½-tablet dose leaves 29.5 of 30). Decremented by the
             intake consumption hook (FEFO with spillover across containers); refunded when a taken dose is skipped,
-            edited away, or deleted.
+            edited away, or deleted. v1.18.3 (iOS#31) — NULL when the count is unknown (a corrupt or legacy row); the
+            client renders "unknown", never a fabricated 0 it could decrement into negatives.
         firstUseAt:
           anyOf:
             - type: string