fix(import): accept a blood-glucose reading that carries no context

A CSV import of a continuous-sensor export wrote nothing and rendered a
green success state. Three faults compounded, and the reporter took all
three apart before filing.

The parser refused every BLOOD_GLUCOSE row with a blank glucoseContext.
Behind it, the CHECK constraint from migration 0021 read both ways at
once: a glucose row MUST carry a context, every other type MUST NOT. The
second half is right and stays. The first was written when the only
glucose writer was the entry form, where a person picks fasting or
post-meal at the moment of the reading. Every machine writer that
arrived since has no such answer per sample, so the Nightscout pull, the
MCP log tool, the Telegram capture and both bulk importers were all
building rows the database then refused.

A missing context now stores as NULL, never as a fifth enum member.
Every per-context reader selects by equality against the four members,
so the reading stays out of the doctor report's fasting rows, the
matching FHIR observations and the per-context targets, while the chart,
the all-time summary and the clinical panel keep it in full. Hand entry
still asks for a context, because at that moment you know it.

The import cards now read the counts rather than the status code.
Nothing written is a failure, a mixed file is a warning, and the refused
rows are grouped by reason with a count instead of fifty identical
lines, with the per-line detail one disclosure away. Both cards render
through one outcome table, so a tick cannot reach a result that wrote
nothing.

Refs #640

docs/api/openapi.yaml

@@ -8004,9 +8004,11 @@ paths:
         10 000 valid rows). Header (order-independent): `type,value,unit,measuredAt[,glucoseContext,notes,externalId]`.
         `measuredAt` must carry an explicit ISO-8601 offset and is bounded (no future beyond a 5-min skew, no instant
         before 1900). Glucose accepts `mmol/L` (converted to canonical `mg/dL`); weight accepts `lb` (converted to
-        `kg`). An `externalId` column makes re-upload idempotent (upsert on `(userId, type, source=IMPORT,
-        externalId)`); without it a re-upload duplicates. `?dryRun=1` validates + previews without writing. Shares the
-        5/hour `import:` rate bucket."
+        `kg`). `glucoseContext` is optional on a `BLOOD_GLUCOSE` row — a blank cell stores no context, which is the
+        normal shape of a continuous-sensor export; a non-empty value is still checked against the enum, and the column
+        is refused on any other type. An `externalId` column makes re-upload idempotent (upsert on `(userId, type,
+        source=IMPORT, externalId)`); without it a re-upload duplicates. `?dryRun=1` validates + previews without
+        writing. Shares the 5/hour `import:` rate bucket."
       parameters:
         - name: dryRun
           in: query
@@ -8027,6 +8029,7 @@ paths:
                 type,value,unit,measuredAt,glucoseContext,notes,externalId
                 WEIGHT,80.5,kg,2026-05-01T08:00:00Z,,morning,
                 BLOOD_GLUCOSE,5.3,mmol/L,2026-05-01T08:05:00+02:00,FASTING,,meter-001
+                BLOOD_GLUCOSE,5.9,mmol/L,2026-05-01T08:20:00+02:00,,,sensor-001
       responses:
         "200":
           description: Per-row import outcome.
@@ -17199,7 +17202,8 @@ components:
             - type: string
             - type: "null"
         glucoseContext:
-          description: Set only on BLOOD_GLUCOSE rows.
+          description: Set only on BLOOD_GLUCOSE rows, and null there too when the writer recorded no classification (a sensor
+            export, a device sync). Never invent one.
           anyOf:
             - type: string
             - type: "null"
@@ -18015,7 +18019,8 @@ components:
             - type: string
             - type: "null"
         glucoseContext:
-          description: Set only on BLOOD_GLUCOSE rows.
+          description: Set only on BLOOD_GLUCOSE rows, and null there too when the writer recorded no classification (a sensor
+            export, a device sync). Never invent one.
           anyOf:
             - type: string
             - type: "null"