Reduce handler request boilerplate

- Add session.Attach to reuse per-request sessions across middleware and ghome handlers
- Make Session finalize/abort paths idempotent to avoid duplicate commit or rollback work
- Move request_data parsing helper to internal/utils
- Expand ParseRequestData usage across api, download, unit, live, multiunit, and subscenario handlers
- Extract download package metadata into internal/handler/download/types.go
- Return explicit errors from selected session lookup helpers and update callers
- Make api action handlers return errors directly instead of calling ss.CheckErr in nested layers

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/handler/live/play.go

@@ -7,6 +7,7 @@ import (
 	"honoka-chan/internal/router"
 	liveschema "honoka-chan/internal/schema/live"
 	"honoka-chan/internal/session"
+	honokautils "honoka-chan/internal/utils"
 	"honoka-chan/pkg/utils"
 	"math"
 	"strconv"
@@ -20,7 +21,7 @@ func play(ctx *gin.Context) {
 	defer ss.Finalize()
 
 	playReq := liveschema.PlayReq{}
-	err := json.Unmarshal([]byte(ctx.MustGet("request_data").(string)), &playReq)
+	err := honokautils.ParseRequestData(ctx, &playReq)
 	if ss.CheckErr(err) {
 		return
 	}

internal/handler/live/precisescore.go

@@ -8,6 +8,7 @@ import (
 	"honoka-chan/internal/router"
 	liveschema "honoka-chan/internal/schema/live"
 	"honoka-chan/internal/session"
+	honokautils "honoka-chan/internal/utils"
 	"honoka-chan/pkg/utils"
 	"strconv"
 	"time"
@@ -20,7 +21,7 @@ func preciseScore(ctx *gin.Context) {
 	defer ss.Finalize()
 
 	playScoreReq := liveschema.PlayScoreReq{}
-	err := json.Unmarshal([]byte(ctx.MustGet("request_data").(string)), &playScoreReq)
+	err := honokautils.ParseRequestData(ctx, &playScoreReq)
 	if ss.CheckErr(err) {
 		return
 	}

internal/handler/live/reward.go

@@ -1,12 +1,12 @@
 package live
 
 import (
-	"encoding/json"
 	"honoka-chan/internal/middleware"
 	usermodel "honoka-chan/internal/model/user"
 	"honoka-chan/internal/router"
 	liveschema "honoka-chan/internal/schema/live"
 	"honoka-chan/internal/session"
+	honokautils "honoka-chan/internal/utils"
 	"honoka-chan/pkg/utils"
 	"time"
 
@@ -21,7 +21,7 @@ func reward(ctx *gin.Context) {
 	}()
 
 	playRewardReq := liveschema.RewardReq{}
-	err := json.Unmarshal([]byte(ctx.MustGet("request_data").(string)), &playRewardReq)
+	err := honokautils.ParseRequestData(ctx, &playRewardReq)
 	if ss.CheckErr(err) {
 		return
 	}