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.

src/lib/openapi/routes/medications.ts

@@ -296,13 +296,15 @@ const medicationInventoryItemResource = z
       ),
     unitsTotal: z
       .number()
+      .nullable()
       .describe(
-        "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`.",
+        "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`. 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: z
       .number()
+      .nullable()
       .describe(
-        "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.",
+        "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. 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: z.iso
       .datetime({ offset: true })