Add Test Connection button to AI settings with plain-English error messages

- POST /settings/ai/test sends a minimal text prompt to verify the provider
- Returns ok/message — maps HTTP status codes to human-readable explanations
- 405 → "URL pointing at wrong place, check custom URL has no path suffix"
- 401 → invalid key, 404 → wrong path, 429 → rate limited, etc.
- Test button appears once a key is saved; result shown inline below buttons

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

backend/app/api/v1/settings.py

@@ -71,6 +71,80 @@ async def save_ai_settings(
     )
 
 
+_STATUS_MEANINGS = {
+    400: "Bad request — the model name may be invalid or the request format is unsupported.",
+    401: "Unauthorised — your API key is incorrect or has been revoked.",
+    403: "Forbidden — your API key doesn't have permission to use this model or endpoint.",
+    404: "Not found — the API URL path is wrong. Check your custom URL doesn't include /v1 (MyMidas adds it automatically).",
+    405: "Method not allowed — the API URL is pointing at the wrong place. For Open WebUI, the base URL should be just http://host:port with no path suffix.",
+    422: "Unprocessable — the model name was rejected. Check it matches exactly what the provider expects.",
+    429: "Rate limited — too many requests. Try again in a moment.",
+    500: "Server error — the AI provider returned an internal error.",
+    502: "Bad gateway — could not reach the AI provider. Check the URL is correct and the service is running.",
+    503: "Service unavailable — the AI provider is down or overloaded.",
+}
+
+
+@router.post("/ai/test")
+async def test_ai_settings(
+    db: AsyncSession = Depends(get_db),
+    user: User = Depends(get_current_user),
+):
+    """Send a minimal test prompt to verify the configured AI provider works."""
+    import httpx
+    from sqlalchemy import select as sa_select
+
+    user_row = await db.get(User, user.id)
+    if not user_row or not user_row.ai_provider or not user_row.ai_api_key_enc:
+        return {"ok": False, "message": "No AI provider configured. Save your settings first."}
+
+    api_key = decrypt_field(user_row.ai_api_key_enc)
+    custom_base_url = (user_row.ai_base_url or "").rstrip("/")
+    custom_model = (user_row.ai_model or "").strip()
+    test_prompt = "Reply with the single word: OK"
+
+    try:
+        if user_row.ai_provider == "anthropic":
+            base_url = custom_base_url or "https://api.anthropic.com"
+            model = custom_model or "claude-haiku-4-5-20251001"
+            async with httpx.AsyncClient(timeout=15) as client:
+                resp = await client.post(
+                    f"{base_url}/v1/messages",
+                    headers={"x-api-key": api_key, "anthropic-version": "2023-06-01", "content-type": "application/json"},
+                    json={"model": model, "max_tokens": 10, "messages": [{"role": "user", "content": test_prompt}]},
+                )
+            resp.raise_for_status()
+            reply = resp.json()["content"][0]["text"].strip()
+
+        elif user_row.ai_provider == "openai":
+            base_url = custom_base_url or "https://api.openai.com"
+            model = custom_model or "gpt-4o-mini"
+            async with httpx.AsyncClient(timeout=15) as client:
+                resp = await client.post(
+                    f"{base_url}/v1/chat/completions",
+                    headers={"Authorization": f"Bearer {api_key}", "content-type": "application/json"},
+                    json={"model": model, "max_tokens": 10, "messages": [{"role": "user", "content": test_prompt}]},
+                )
+            resp.raise_for_status()
+            reply = resp.json()["choices"][0]["message"]["content"].strip()
+
+        else:
+            return {"ok": False, "message": "Unknown provider."}
+
+        return {"ok": True, "message": f'Connection successful. Model replied: "{reply}"'}
+
+    except httpx.HTTPStatusError as e:
+        code = e.response.status_code
+        meaning = _STATUS_MEANINGS.get(code, f"HTTP {code} error.")
+        return {"ok": False, "message": f"HTTP {code}: {meaning}"}
+    except httpx.ConnectError:
+        return {"ok": False, "message": "Connection refused — the URL is unreachable. Check the host and port are correct and the service is running."}
+    except httpx.TimeoutException:
+        return {"ok": False, "message": "Request timed out — the server didn't respond within 15 seconds."}
+    except httpx.RequestError as e:
+        return {"ok": False, "message": f"Network error: {e}"}
+
+
 @router.delete("/ai", status_code=204)
 async def clear_ai_settings(
     db: AsyncSession = Depends(get_db),