feat(fhir): serve Observations as a time series, code RMSSD and workouts

GET /api/fhir/Observation keeps its snapshot answer when no date is
sent. A date range (FHIR prefixes eq/gt/ge/lt/le, UTC when no offset)
switches it to the time series: every reading in the range plus
workouts, newest first, paged in the database through one UNION id
query and a count, so a page costs what it holds. Series ids are the
row's own (m-<id>, w-<id>) and read back through /Observation/{id}.
The series replays the same saved report selection as the snapshot,
pairs systolic and diastolic recorded at one instant into the
blood-pressure panel, leaves sleep stage segments out, and reads an
hourly heart-rate bucket as a mean over its hour. A code parameter
narrows either answer; a malformed date is a 400 OperationOutcome.

A workout is an activity Observation after the Physical Activity IG:
LOINC 73985-4 with the sport as its value and duration (55411-3) and
energy (41981-2) as components. RMSSD gets its own registry coding,
text-only because LOINC has no RMSSD term, so it never borrows the SDNN
code. The capability statement lists the two new parameters.

docs/api/openapi.yaml

@@ -1245,8 +1245,9 @@ paths:
       summary: FHIR R4 CapabilityStatement (v1.11.0)
       description: "Read-only FHIR R4 capability statement for the REST face. Declares the served resource types (Patient,
         Coverage, Observation, MedicationStatement, MedicationAdministration), their `read` and `search-type`
-        interactions, the `Patient/$everything` operation, and the `application/fhir+json` format. Every advertised
-        interaction has a route. Auth: `fhir:read` scope (cookie sessions also pass)."
+        interactions, the search parameters (`_count`, `_offset`; `date` and `code` on Observation), the
+        `Patient/$everything` operation, and the `application/fhir+json` format. Every advertised interaction has a
+        route. Auth: `fhir:read` scope (cookie sessions also pass)."
       responses:
         "200":
           description: CapabilityStatement (application/fhir+json).
@@ -1291,7 +1292,16 @@ paths:
         - FHIR
       summary: FHIR R4 Observation search (v1.11.0)
       description: "Read-only `searchset` Bundle of the caller's own Observations (vitals / activity / lab / survey). Auth:
-        `fhir:read` scope. Offset paging via `_count` (clamped ≤200) / `_offset`."
+        `fhir:read` scope. Offset paging via `_count` (clamped ≤200) / `_offset`. Without `date` the answer is the
+        snapshot it has always been: one latest reading per type plus the derived panels, ids `obs-N`. With `date`
+        (v1.44, repeatable, FHIR prefixes eq/gt/ge/lt/le, a value without an offset read as UTC) the answer is the time
+        series instead: every reading in the range plus workouts, newest first, paged in the database, ids `m-<id>` /
+        `w-<id>` that read back through `/api/fhir/Observation/{id}`. The series follows the same saved report selection
+        as the snapshot, leaves sleep out (stored as stage segments), pairs a systolic and diastolic recorded at the
+        same instant into the blood-pressure panel, and codes a workout as an `activity` Observation (LOINC 73985-4
+        Exercise activity, duration 55411-3 and energy 41981-2 as components). `code` (repeatable, `system|code` or a
+        bare code, comma-separated) narrows either answer; an unknown code matches nothing. A malformed `date` is a 400
+        OperationOutcome."
       parameters:
         - in: query
           name: _count
@@ -1301,6 +1311,14 @@ paths:
           name: _offset
           schema:
             type: number
+        - in: query
+          name: date
+          schema:
+            type: string
+        - in: query
+          name: code
+          schema:
+            type: string
       responses:
         "200":
           description: searchset Bundle (application/fhir+json).
@@ -1463,9 +1481,10 @@ paths:
       tags:
         - FHIR
       summary: FHIR R4 Observation read
-      description: "Read-only `Observation` by id, out of the caller's own record. An id belonging to someone else is
-        indistinguishable from one that never existed — both answer a `not-found` OperationOutcome. Auth: `fhir:read`
-        scope; `userId` is narrowed from auth."
+      description: "Read-only `Observation` by id, out of the caller's own record: a snapshot id (`obs-N`) or a series id
+        (`m-<id>` / `w-<id>`, v1.44), the latter read inside the same saved selection the series search applies. An id
+        belonging to someone else is indistinguishable from one that never existed — both answer a `not-found`
+        OperationOutcome. Auth: `fhir:read` scope; `userId` is narrowed from auth."
       parameters:
         - in: path
           name: id