feat(coach): document chats live in the Coach view and history

Asking the Coach about a document opened a separate, half-finished chat panel
that never appeared in Coach history. The bespoke drawer is gone; a document
conversation now opens the real Coach view, appears in the Coach list with a
document badge and a "Chatting about: <title>" note, and is read back through
the same history rail as any other thread.

The prompt-injection fence is unchanged and stays server-side. A document turn
is still sent through the hardened fenced endpoint (no tools, no health
snapshot, grounded in the document text only); the coach tool route continues
to fetch with documentId null, so a document-scoped conversation can never be
continued through the tool loop even from a hostile client. Only the surface
moved into the Coach; the boundary did not.

The relaxed conversation-list and detail filters drop only the documentId
predicate — userId scoping is unchanged, so a document thread is still visible
only to its owner. CoachConversationDTO gains documentId + documentTitle
(server-derived, title/filename only); no schema change (documentId exists
since 0230). OpenAPI regenerated.

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.28.50
+  version: 1.28.51
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.
 
@@ -21934,6 +21934,18 @@ components:
           type: integer
           minimum: -9007199254740991
           maximum: 9007199254740991
+        documentId:
+          description: When set, this thread is scoped to one stored document (a document chat, run on the hardened fenced
+            endpoint). Null/absent for a normal health thread.
+          anyOf:
+            - type: string
+            - type: "null"
+        documentTitle:
+          description: The owning document's resolved title (title, falling back to filename) for the scope badge. Null on a
+            health thread or when the document has neither.
+          anyOf:
+            - type: string
+            - type: "null"
       required:
         - id
         - title
@@ -21988,6 +22000,17 @@ components:
           anyOf:
             - type: string
             - type: "null"
+        documentId:
+          description: When set, this thread is scoped to one stored document; the client routes its turns through the hardened
+            fenced document endpoint, never the tool route.
+          anyOf:
+            - type: string
+            - type: "null"
+        documentTitle:
+          description: The owning document's resolved title for the scope badge. Null on a health thread.
+          anyOf:
+            - type: string
+            - type: "null"
       required:
         - id
         - title