fix(api): constrain client-writable sources and batch timestamps

Narrow the workout create schema's source to the client-writable set
(MANUAL, APPLE_HEALTH), matching the measurement write surfaces, so a
client cannot forge a server-owned source into the cross-source picker.
Apply the single-entry plausibility bound to the measurement batch
timestamps so a backdated or future-dated row can no longer slip past
the batch path. Regenerate the OpenAPI contract.

docs/api/openapi.yaml

@@ -2664,17 +2664,7 @@ paths:
                         type: string
                         enum:
                           - MANUAL
-                          - WITHINGS
-                          - IMPORT
                           - APPLE_HEALTH
-                          - COMPUTED
-                          - WHOOP
-                          - FITBIT
-                          - NIGHTSCOUT
-                          - POLAR
-                          - OURA
-                          - TELEGRAM
-                          - MCP
                       externalId:
                         type: string
                         maxLength: 128