backend: render patient card from searchPatients on a unique match

"Show me <name>'s medical record" relied on the model chaining
searchPatients → getPatient, but Gemini Flash often calls searchPatients
then emits the canned closing line ("Here's the record.") without the
second tool call, so no data-patientCard part is ever written and no card
renders. (The prior Gemini fix only covered the empty-schema list tools.)

searchPatients now writes the record card (data-patientCard, or
data-recordGraph in graph mode) and a source directly when EXACTLY ONE
patient matches — mirroring getPatient — so the common name-lookup flow no
longer depends on a second tool call. Multiple/zero matches keep returning
the disambiguation list. The tool description and system prompt tell the
model the card is already shown on a unique match so it doesn't double-render.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/routes/chat.ts

@@ -107,7 +107,10 @@ function systemPrompt(
     "",
     "Display tools (read-only):",
     "- getPatient: when asked about a specific patient by file number / MRN.",
-    "- searchPatients: when given a name; then getPatient on the match.",
+    "- searchPatients: when given a name. If exactly one patient matches it",
+    "  already shows that patient's record card — don't call getPatient again,",
+    "  just confirm. Only call getPatient yourself for a direct file number / MRN,",
+    "  or to pick one of several matches it returns.",
     "- getPatientLabs: when asked about labs/results/trends.",
     "- listAppointments: when asked to see the schedule / upcoming visits.",
     "- listTasks: when asked to see open tasks / to-dos.",