refactor(api): separate business logic into game, tournament, session packages

Extract business logic from api/handler.go into dedicated service packages:
- session: context helpers (resolves admin → api import dependency)
- game: game state, code submission, ranking, watch logic
- tournament: bracket construction and seed ordering
- api/convert.go: domain → API type conversion functions

api/handler.go is now a thin adapter that delegates to services and
maps domain errors to HTTP status codes.

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

1 files changed, 97 insertions, 0 deletions

diff --git a/backend/tournament/service_test.go b/backend/tournament/service_test.go
new file mode 100644
index 0000000..d1ca78c
--- /dev/null
+++ b/backend/tournament/service_test.go
@@ -0,0 +1,97 @@
+package tournament
+
+import "testing"
+
+func TestStandardBracketSeeds(t *testing.T) {
+	tests := []struct {
+		name        string
+		bracketSize int
+		expected    []int
+	}{
+		{
+			name:        "bracket_size=2",
+			bracketSize: 2,
+			expected:    []int{1, 2},
+		},
+		{
+			name:        "bracket_size=4",
+			bracketSize: 4,
+			expected:    []int{1, 4, 2, 3},
+		},
+		{
+			name:        "bracket_size=8",
+			bracketSize: 8,
+			expected:    []int{1, 8, 4, 5, 2, 7, 3, 6},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := StandardBracketSeeds(tt.bracketSize)
+			if len(got) != len(tt.expected) {
+				t.Fatalf("expected length %d, got %d", len(tt.expected), len(got))
+			}
+			for i, v := range tt.expected {
+				if got[i] != v {
+					t.Errorf("position %d: expected seed %d, got %d", i, v, got[i])
+				}
+			}
+		})
+	}
+}
+
+func TestStandardBracketSeeds_Seed1And2OppositeSides(t *testing.T) {
+	seeds := StandardBracketSeeds(8)
+	seed1Pos := -1
+	seed2Pos := -1
+	for i, s := range seeds {
+		if s == 1 {
+			seed1Pos = i
+		}
+		if s == 2 {
+			seed2Pos = i
+		}
+	}
+	if seed1Pos >= 4 {
+		t.Errorf("Seed 1 should be in first half, but at position %d", seed1Pos)
+	}
+	if seed2Pos < 4 {
+		t.Errorf("Seed 2 should be in second half, but at position %d", seed2Pos)
+	}
+}
+
+func TestStandardBracketSeeds_AllSeedsPresent(t *testing.T) {
+	for _, size := range []int{2, 4, 8, 16} {
+		seeds := StandardBracketSeeds(size)
+		seen := make(map[int]bool)
+		for _, s := range seeds {
+			if s < 1 || s > size {
+				t.Errorf("bracket_size=%d: seed %d out of range", size, s)
+			}
+			if seen[s] {
+				t.Errorf("bracket_size=%d: duplicate seed %d", size, s)
+			}
+			seen[s] = true
+		}
+		if len(seen) != size {
+			t.Errorf("bracket_size=%d: expected %d unique seeds, got %d", size, size, len(seen))
+		}
+	}
+}
+
+func TestFindSeedByUserID(t *testing.T) {
+	entries := []TournamentEntry{
+		{User: Player{UserID: 10}, Seed: 1},
+		{User: Player{UserID: 20}, Seed: 2},
+		{User: Player{UserID: 30}, Seed: 3},
+	}
+
+	if got := findSeedByUserID(entries, 10); got != 1 {
+		t.Errorf("expected seed 1 for user 10, got %d", got)
+	}
+	if got := findSeedByUserID(entries, 20); got != 2 {
+		t.Errorf("expected seed 2 for user 20, got %d", got)
+	}
+	if got := findSeedByUserID(entries, 999); got != 0 {
+		t.Errorf("expected seed 0 for unknown user, got %d", got)
+	}
+}