Cache economic calendar for 1 hour, fix SCAN_BUTTONS test for EUR-only

The weekly calendar JSON was re-fetched on every scan cycle (~288
calls/day). Now cached per-instance with 1-hour TTL, cutting to ~24.
Currency filtering still applied on each read so cached events stay
correct across pairs.

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

test_all.py

@@ -1475,7 +1475,7 @@ def _():
 
 @test("SCAN_BUTTONS and SCAN_LOOKUP consistency")
 def _():
-    assert len(SCAN_BUTTONS) == 10  # 5 per pair (4 TFs + All) × 2 pairs
+    assert len(SCAN_BUTTONS) == 5  # 4 TFs + All for EUR/USD
     for btn in SCAN_BUTTONS:
         assert btn["cb"] in SCAN_LOOKUP
         assert "symbol" in btn