perf(illness): compute the recovery gap lazily and gate it on signal density

The illness list load triggered the cross-episode correlation fan-out on
every mount, so the page waited on up to 24 per-episode reads before it
painted. Move that work behind an explicit opt-in: the insights endpoint
takes an includeRecoveryGap query param (default off), the list reads the
count breakdown on a single fast query, and the recovery gap is computed
only when the viewer opens the Analyse expansion.

Tighten the typical-gap gate against coincidental data. A gap now counts
only from an episode that cleared a per-episode measurement floor
(MIN_GAP_MEASUREMENT_DAYS), at least MIN_EPISODES_FOR_TYPICAL_GAP such
episodes must qualify, and the median magnitude must reach
MIN_TYPICAL_GAP_MAGNITUDE_DAYS — a 0/+-1-day gap is the coincidental
baseline and is withheld as noise.

docs/api/openapi.yaml

@@ -6904,6 +6904,13 @@ paths:
             type: integer
             minimum: 30
             maximum: 1095
+        - in: query
+          name: includeRecoveryGap
+          schema:
+            type: string
+            enum:
+              - "true"
+              - "false"
       responses:
         "200":
           description: The cross-episode retrospective summary.