feat(coach): surface per-turn token usage on the chat stream

Carry an additive usage envelope on the SSE done frame
({ totalTokens, model }) and persist tokensUsed + model onto
coach_messages (migration 0182, nullable, no backfill) so the
per-message token footer survives a conversation reload. Add an
optional reasoning frame the client can render inside the thinking
disclosure when a reasoning-capable provider emits a summary; providers
without reasoning emit none. Extend CoachMessageDTO and regenerate the
OpenAPI contract. Server-authoritative: clients display the count, never
recompute it.

docs/api/openapi.yaml

@@ -4148,11 +4148,14 @@ paths:
       summary: Send a Coach turn (streaming reply)
       description: "v1.18.0 — sends a user turn and streams the assistant reply as Server-Sent Events. The response is
         `text/event-stream`, not JSON: one `data: <json>\\n\\n` frame per event. Frame `type` is one of `token` (a chunk
-        of reply text: `{ type, token }`), `provenance` (the evidence envelope: `{ type, metricSource }`), `done` (`{
-        type, conversationId, messageId }`), or `error` (`{ type, code, message }`). The HTTP status is 200 even for a
-        provider/refusal outcome — clients dispatch on the `error` frame, not the status. Clients ignore unknown frame
-        types (additive evolution). Omitting `conversationId` starts a new conversation. Coach-gated; budget- and
-        rate-limited. Auth via cookie or Bearer."
+        of reply text: `{ type, token }`), `provenance` (the evidence envelope: `{ type, metricSource }`), `suggestion`
+        (a cadence-suggestion card: `{ type, suggestion }`), `reasoning` (v1.18.9, optional reasoning-summary text: `{
+        type, text }` — emitted only by reasoning-capable providers; absent otherwise), `done` (`{ type, conversationId,
+        messageId, usage? }` — v1.18.9 adds the optional `usage` envelope `{ totalTokens, promptTokens?,
+        completionTokens?, model }`, server-authoritative; clients display it, never recompute), or `error` (`{ type,
+        code, message }`). The HTTP status is 200 even for a provider/refusal outcome — clients dispatch on the `error`
+        frame, not the status. Clients ignore unknown frame types (additive evolution). Omitting `conversationId` starts
+        a new conversation. Coach-gated; budget- and rate-limited. Auth via cookie or Bearer."
       requestBody:
         required: true
         content:
@@ -15702,6 +15705,20 @@ components:
             - type: string
             - type: "null"
           description: Coach prompt version that produced the reply; null for user turns.
+        tokensUsed:
+          anyOf:
+            - type: integer
+              minimum: -9007199254740991
+              maximum: 9007199254740991
+            - type: "null"
+          description: v1.18.9 — total tokens this assistant turn cost, persisted so the per-message token footer survives a
+            reload. Null on user turns, refusals, and pre-feature rows.
+        model:
+          anyOf:
+            - type: string
+            - type: "null"
+          description: v1.18.9 — the provider model that produced the reply (e.g. gpt-4o). Null when unknown (user turns,
+            refusals, older rows).
       required:
         - id
         - role
@@ -15710,6 +15727,8 @@ components:
         - metricSource
         - providerType
         - promptVersion
+        - tokensUsed
+        - model
       additionalProperties: false
       description: One Coach chat message, decrypted server-side. Ordered oldest-first within a conversation.
     CoachProvenance: