feat(fees): junior rate for 2026-08 is 0

- 2026-08 added to the junior rate table at 0 CZK, same as adults
- A single practice in a 0-rate month is 0 instead of "?", since a
  free month needs no manual review
- Mirrored in Go and legacy Python; new calculate_junior_fee parity
  fixtures

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

go/tests/fixtures/pure/calculate_junior_fee/one_session_free_august.json

@@ -0,0 +1,13 @@
+{
+  "case": "one_session_free_august",
+  "func": "scripts.attendance.calculate_junior_fee",
+  "captured_at": "2026-10-08",
+  "input": {
+    "attendance_count": 1,
+    "month_key": "2026-08"
+  },
+  "output": {
+    "value": 0,
+    "unknown": false
+  }
+}

go/tests/fixtures/pure/calculate_junior_fee/two_sessions_free_august.json

@@ -0,0 +1,13 @@
+{
+  "case": "two_sessions_free_august",
+  "func": "scripts.attendance.calculate_junior_fee",
+  "captured_at": "2026-10-08",
+  "input": {
+    "attendance_count": 2,
+    "month_key": "2026-08"
+  },
+  "output": {
+    "value": 0,
+    "unknown": false
+  }
+}