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/one_session_free_july.json

@@ -0,0 +1,12 @@
+{
+  "case": "one_session_free_july",
+  "func": "scripts.attendance.calculate_fee",
+  "captured_at": "2026-10-08",
+  "input": {
+    "attendance_count": 1,
+    "month_key": "2026-07"
+  },
+  "output": {
+    "fee": 0
+  }
+}

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

@@ -1,12 +1,12 @@
 {
   "case": "two_sessions_default_fallback",
   "func": "scripts.attendance.calculate_fee",
-  "captured_at": "2026-05-06",
+  "captured_at": "2026-10-08",
   "input": {
     "attendance_count": 2,
     "month_key": "2099-01"
   },
   "output": {
-    "fee": 700
+    "fee": 600
   }
 }

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

@@ -0,0 +1,12 @@
+{
+  "case": "two_sessions_free_july",
+  "func": "scripts.attendance.calculate_fee",
+  "captured_at": "2026-10-08",
+  "input": {
+    "attendance_count": 2,
+    "month_key": "2026-07"
+  },
+  "output": {
+    "fee": 0
+  }
+}

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
+  }
+}