feat(fees): adult rates for 2026-07 to 2026-12, default 600

- Default adult monthly rate 700 -> 600 CZK
- 2026-07 and 2026-08 set to 0; 2026-09 through 2026-12 added at 600
- Single-session fee is capped at the month's rate, so a 0-rate month
  is free for one session as well
- Mirrored in Go and legacy Python; new calculate_fee parity fixtures

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go/tests/fixtures/pure/calculate_fee/two_sessions_october.json

@@ -0,0 +1,12 @@
+{
+  "case": "two_sessions_october",
+  "func": "scripts.attendance.calculate_fee",
+  "captured_at": "2026-10-08",
+  "input": {
+    "attendance_count": 2,
+    "month_key": "2026-10"
+  },
+  "output": {
+    "fee": 600
+  }
+}