Add per-service system prompts with AI Settings tab view

Each feature service owns its system prompt in its config JSON on the
shared volume. The AI Settings page now has General and System Prompts
tabs — admins can view and edit any service's prompts at runtime with
changes taking effect within 30 s (config cache TTL).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/settings.py

@@ -11,13 +11,16 @@ from fastapi import APIRouter, Depends, HTTPException
 from pydantic import BaseModel
 
 from app.core.app_config import (
+    SYSTEM_PROMPT_SERVICES,
     _merge_api_key,
     load_ai_service_config,
     load_ai_service_config_masked,
+    load_all_system_prompts,
     load_doc_service_config,
     load_doc_service_config_masked,
     save_ai_service_config,
     save_doc_service_config,
+    save_service_system_prompts,
 )
 from app.core.config import settings
 from app.deps import get_current_admin
@@ -45,6 +48,11 @@ class LimitsUpdate(BaseModel):
     max_pdf_mb: int
 
 
+class SystemPromptUpdate(BaseModel):
+    system: str
+    user_template: str
+
+
 # ── AI settings ────────────────────────────────────────────────────────────────
 
 
@@ -142,3 +150,29 @@ async def update_documents_limits(
     config.documents.max_pdf_bytes = body.max_pdf_mb * 1024 * 1024
     await asyncio.to_thread(save_doc_service_config, config)
     return load_doc_service_config_masked()
+
+
+# ── System prompts ─────────────────────────────────────────────────────────────
+
+
+@router.get("/system-prompts")
+async def get_system_prompts(
+    _: User = Depends(get_current_admin),
+) -> dict:
+    """Return all editable system prompts, keyed by service id."""
+    return await asyncio.to_thread(load_all_system_prompts)
+
+
+@router.patch("/system-prompts/{service_id}")
+async def update_system_prompt(
+    service_id: str,
+    body: SystemPromptUpdate,
+    _: User = Depends(get_current_admin),
+) -> dict:
+    """Update the system prompts for a single service."""
+    if service_id not in SYSTEM_PROMPT_SERVICES:
+        raise HTTPException(status_code=404, detail=f"No system prompts registered for {service_id!r}")
+    await asyncio.to_thread(
+        save_service_system_prompts, service_id, body.system, body.user_template
+    )
+    return await asyncio.to_thread(load_all_system_prompts)