feat(mcp): add a dedicated MCP measurement source for write provenance

Append an MCP value to MeasurementSource (migration 0196, additive enum
extension) so a row logged through the remote MCP endpoint is distinguishable
from MANUAL/TELEGRAM provenance and gets an isolated idempotency namespace
(userId, type, MCP, externalId). The value joins the read enum so existing
read/response shapes decode the rows; the REST batch and single-POST write
surfaces still reject it (server-resolved only). Regenerate the OpenAPI
contract for the additive enum.

src/lib/validations/measurement.ts

@@ -184,6 +184,13 @@ export const measurementSourceEnum = z.enum([
   // decode the rows it surfaces; the client-facing write surfaces still
   // reject it (see `WRITABLE_MEASUREMENT_SOURCES` + the batch allowlist).
   "TELEGRAM",
+  // v1.22.0 — measurements logged through the confirmed remote MCP write
+  // surface under a `health:write`-scoped token. Server-resolved (the row is
+  // written in-process over `/mcp`, never through the cookie/Bearer client
+  // write path), so it is excluded from `WRITABLE_MEASUREMENT_SOURCES`; part
+  // of this enum so the read/response shapes (and the iOS decoder) can decode
+  // the rows it surfaces.
+  "MCP",
 ]);
 
 /**