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, 39 insertions, 0 deletions

diff --git a/backend/session/context.go b/backend/session/context.go
new file mode 100644
index 0000000..60f88b4
--- /dev/null
+++ b/backend/session/context.go
@@ -0,0 +1,39 @@
+package session
+
+import (
+	"context"
+
+	"albatross-2026-backend/db"
+)
+
+type sessionIDContextKey struct{}
+type userContextKey struct{}
+type clientIPContextKey struct{}
+
+func GetSessionIDFromContext(ctx context.Context) (string, bool) {
+	sessionID, ok := ctx.Value(sessionIDContextKey{}).(string)
+	return sessionID, ok
+}
+
+func SetSessionIDInContext(ctx context.Context, sessionID string) context.Context {
+	return context.WithValue(ctx, sessionIDContextKey{}, sessionID)
+}
+
+func GetUserFromContext(ctx context.Context) (*db.User, bool) {
+	user, ok := ctx.Value(userContextKey{}).(*db.User)
+	return user, ok
+}
+
+// SetUserInContext sets a user in the context. Intended for testing.
+func SetUserInContext(ctx context.Context, user *db.User) context.Context {
+	return context.WithValue(ctx, userContextKey{}, user)
+}
+
+func GetClientIPFromContext(ctx context.Context) string {
+	ip, _ := ctx.Value(clientIPContextKey{}).(string)
+	return ip
+}
+
+func SetClientIPInContext(ctx context.Context, ip string) context.Context {
+	return context.WithValue(ctx, clientIPContextKey{}, ip)
+}