feat(ui): add real-time diagnostic logs console (#974)

* feat(diagnostics): add admin-gated real-time diagnostics logs terminal UI

* feat(ui): resolve diagnostics logs feedback and optimize client-side caching

* feat(ui): resolve diagnostics logs feedback

tests/test_diagnostics_logs.py

@@ -0,0 +1,110 @@
+"""Route-level regression tests for GET /api/diagnostics/logs."""
+
+import pytest
+
+fastapi = pytest.importorskip("fastapi")
+pytest.importorskip("starlette.testclient")
+
+from fastapi import FastAPI, HTTPException, Request
+from starlette.testclient import TestClient
+
+# Importing the route module pulls a few app deps; skip cleanly if unavailable.
+diag = pytest.importorskip("routes.diagnostics_routes")
+
+
+def _client_with_admin_gate(monkeypatch, gate, tmp_path=None):
+    """Mount the diagnostics router with a mock require_admin and DATA_DIR."""
+    monkeypatch.setattr(diag, "require_admin", gate)
+    if tmp_path:
+        monkeypatch.setattr(diag, "DATA_DIR", str(tmp_path))
+
+    app = FastAPI()
+    app.include_router(diag.setup_diagnostics_routes(
+        rag_manager=None, rag_available=False, research_handler=None,
+        memory_vector=None))
+    return TestClient(app, raise_server_exceptions=False)
+
+
+def test_logs_unauthenticated_rejected(monkeypatch):
+    def gate(_request: Request):
+        raise HTTPException(401, "Not authenticated")
+    client = _client_with_admin_gate(monkeypatch, gate)
+    r = client.get("/api/diagnostics/logs")
+    assert r.status_code == 401
+
+
+def test_logs_non_admin_forbidden(monkeypatch):
+    def gate(_request: Request):
+        raise HTTPException(403, "Admin only")
+    client = _client_with_admin_gate(monkeypatch, gate)
+    r = client.get("/api/diagnostics/logs")
+    assert r.status_code == 403
+
+
+def test_logs_missing_file(monkeypatch, tmp_path):
+    def gate(_request: Request):
+        return None
+    client = _client_with_admin_gate(monkeypatch, gate, tmp_path)
+    r = client.get("/api/diagnostics/logs")
+    assert r.status_code == 200
+    body = r.json()
+    assert body["status"] == "success"
+    assert body["logs"] == []
+
+
+def test_logs_tailing_and_clamping(monkeypatch, tmp_path):
+    # Setup mock log file
+    log_dir = tmp_path / "logs"
+    log_dir.mkdir(parents=True, exist_ok=True)
+    log_file = log_dir / "app.log"
+
+    # Write 1500 log lines
+    lines = [f"Log line {i}\n" for i in range(1, 1501)]
+    log_file.write_text("".join(lines), encoding="utf-8")
+
+    def gate(_request: Request):
+        return None
+    client = _client_with_admin_gate(monkeypatch, gate, tmp_path)
+
+    # 1. Default limit (200)
+    r = client.get("/api/diagnostics/logs")
+    assert r.status_code == 200
+    body = r.json()
+    assert len(body["logs"]) == 200
+    assert body["logs"][-1] == "Log line 1500"
+    assert body["logs"][0] == "Log line 1301"
+
+    # 2. Clamped upper bound (limit=2000 -> clamps to 1000)
+    r = client.get("/api/diagnostics/logs?limit=2000")
+    assert r.status_code == 200
+    body = r.json()
+    assert len(body["logs"]) == 1000
+    assert body["logs"][-1] == "Log line 1500"
+    assert body["logs"][0] == "Log line 501"
+
+    # 3. Clamped lower bound (limit=-5 -> clamps to 1)
+    r = client.get("/api/diagnostics/logs?limit=-5")
+    assert r.status_code == 200
+    body = r.json()
+    assert len(body["logs"]) == 1
+    assert body["logs"][0] == "Log line 1500"
+
+    # 4. Clamp limit=0 -> clamps to 1
+    r = client.get("/api/diagnostics/logs?limit=0")
+    assert r.status_code == 200
+    body = r.json()
+    assert len(body["logs"]) == 1
+    assert body["logs"][0] == "Log line 1500"
+
+    # 5. Exact custom limit
+    r = client.get("/api/diagnostics/logs?limit=5")
+    assert r.status_code == 200
+    body = r.json()
+    assert len(body["logs"]) == 5
+    assert body["logs"] == [
+        "Log line 1496",
+        "Log line 1497",
+        "Log line 1498",
+        "Log line 1499",
+        "Log line 1500"
+    ]