feat(visits): resolve where the window before a booked visit starts

The visit preparation and the doctor report offered for an appointment
started at the last visit of any kind. An internist one week and a
dermatologist the next gave the dermatologist a one-week window of
nothing it is about.

The list read now publishes `preparation` on every booked upcoming
visit: the first day of the window, the rule that chose it and the
earlier visit it is the day of. First match wins over the visits that
took place before it: the same practitioner, else a practitioner of
the same specialty (free text, compared without case or spacing), else
the same kind for routine, preventive and therapy visits, else the last
90 days. One query of past visits per page, independent of the list's
own filters. Null on every other row; absent on the detail and on
write responses.

docs/api/openapi.yaml

@@ -71911,6 +71911,12 @@ components:
           $ref: "#/components/schemas/EncounterLinks"
         skipped:
           $ref: "#/components/schemas/EncounterSkipReport"
+        preparation:
+          description: "On the list read only (`GET /api/encounters`): the window before this visit when it is booked (PLANNED,
+            upcoming), null on every other row. Absent on the detail and on write responses."
+          anyOf:
+            - $ref: "#/components/schemas/EncounterPreparation"
+            - type: "null"
         createdAt:
           type: string
         updatedAt:
@@ -72062,6 +72068,73 @@ components:
         checkup, which is a measurements-domain write a health-background delegate cannot perform. The visit still
         saves; the checkup stays due and the response says so, because somebody who believes a checkup was closed will
         not look at it again."
+    EncounterPreparation:
+      type: object
+      properties:
+        from:
+          type: string
+          description: "First day of the window, `YYYY-MM-DD` on the record's own calendar: the day of `since`, or 90 days before
+            today on `default`."
+        basis:
+          type: string
+          enum:
+            - practitioner
+            - specialty
+            - kind
+            - default
+          description: "The rule that chose `from`, first match wins over the visits that took place (DONE) before this one:
+            `practitioner` (the last visit with the same practitioner), `specialty` (the last visit with a practitioner
+            of the same specialty, compared without case or extra spaces), `kind` (the last visit of the same kind, for
+            ROUTINE, PREVENTIVE and THERAPY only), `default` (none matched: the last 90 days)."
+        since:
+          anyOf:
+            - $ref: "#/components/schemas/EncounterPreparationAnchor"
+            - type: "null"
+      required:
+        - from
+        - basis
+        - since
+      additionalProperties: false
+      description: "The window before a booked visit, resolved server-side: where the visit preparation and the doctor report
+        offered for the appointment start, and why there. Never the last visit of any kind, which is how a dermatology
+        appointment was offered a week of data from an internist visit. A client shows `from` with its reason and lets
+        the person change the dates; it does not recompute the rule."
+    EncounterPreparationAnchor:
+      type: object
+      properties:
+        id:
+          type: string
+        occurredAt:
+          type: string
+        kind:
+          type: string
+          enum:
+            - ROUTINE
+            - ACUTE
+            - SPECIALIST
+            - PREVENTIVE
+            - EMERGENCY
+            - HOSPITAL
+            - THERAPY
+            - OTHER
+            - PROCEDURE
+        practitionerName:
+          type:
+            - string
+            - "null"
+        specialty:
+          type:
+            - string
+            - "null"
+      required:
+        - id
+        - occurredAt
+        - kind
+        - practitionerName
+        - specialty
+      additionalProperties: false
+      description: "The earlier visit the window starts at: its id, instant and kind, and the practitioner's name and
+        specialty as they read now."
     CreateEncounterEnvelope:
       type: object
       properties: