feat(cache): add read-through cache layer across all app services

Introduce a generic read-through caching infrastructure and wire it into
all major tenant app services to reduce database load on hot read paths.

Key changes:
- Add `DeletePrefix` to Cache interface with memory (prefix scan) and
  Redis (SCAN + DEL) implementations
- New `readthrough.go`: generic `LoadJSON` / `LoadJSONWithEmpty` helpers
  backed by singleflight to prevent cache stampedes; supports jittered TTL
- New `cache_support.go`: centralized cache key builders and invalidation
  helpers for all entities (workspace, brand, prompt rules, schedule tasks,
  articles)
- Wire optional cache into ArticleService, BrandService, WorkspaceService,
  PromptRuleService, ScheduleTaskService, TemplateService, MediaService,
  PromptGenerateService via `WithCache()` builder pattern
- ScheduleDispatchWorker invalidates schedule task cache after dispatching
- ArticleService gains a new `Detail` endpoint with empty-result caching
- Update cmd entrypoints and transport handlers to propagate cache

server/internal/tenant/repository/template_cache_repo.go

@@ -2,18 +2,24 @@ package repository
 
 import (
 	"context"
-	"encoding/json"
+	"errors"
 	"fmt"
 	"time"
 
 	"github.com/geo-platform/tenant-api/internal/shared/cache"
+	"github.com/jackc/pgx/v5"
+	"golang.org/x/sync/singleflight"
 )
 
-const templateCacheTTL = 5 * time.Minute
+const (
+	templateCacheTTL      = 5 * time.Minute
+	templateCacheEmptyTTL = 1 * time.Minute
+)
 
 type cachedTemplateRepository struct {
 	inner TemplateRepository
 	cache cache.Cache
+	group singleflight.Group
 }
 
 func NewCachedTemplateRepository(inner TemplateRepository, c cache.Cache) TemplateRepository {
@@ -23,24 +29,16 @@ func NewCachedTemplateRepository(inner TemplateRepository, c cache.Cache) Templa
 func (r *cachedTemplateRepository) ListTemplates(ctx context.Context, tenantID int64) ([]TemplateRecord, error) {
 	key := fmt.Sprintf("tmpl_list:%d", tenantID)
 
-	if data, err := r.cache.Get(ctx, key); err == nil {
-		var records []TemplateRecord
-		if err := json.Unmarshal(data, &records); err == nil {
-			for i := range records {
-				applyPlatformTemplatePromptOverrides(&records[i])
-			}
-			return records, nil
+	records, _, err := cache.LoadJSONWithEmpty(ctx, r.cache, &r.group, key, templateCacheTTL, templateCacheEmptyTTL, func(loadCtx context.Context) ([]TemplateRecord, bool, error) {
+		records, err := r.inner.ListTemplates(loadCtx, tenantID)
+		if err != nil {
+			return nil, false, err
 		}
-	}
-
-	records, err := r.inner.ListTemplates(ctx, tenantID)
+		return records, true, nil
+	})
 	if err != nil {
 		return nil, err
 	}
-
-	if data, err := json.Marshal(records); err == nil {
-		_ = r.cache.Set(ctx, key, data, templateCacheTTL)
-	}
 	for i := range records {
 		applyPlatformTemplatePromptOverrides(&records[i])
 	}
@@ -50,21 +48,21 @@ func (r *cachedTemplateRepository) ListTemplates(ctx context.Context, tenantID i
 func (r *cachedTemplateRepository) GetTemplateByID(ctx context.Context, id, tenantID int64) (*TemplateRecord, error) {
 	key := fmt.Sprintf("tmpl:%d:%d", tenantID, id)
 
-	if data, err := r.cache.Get(ctx, key); err == nil {
-		var record TemplateRecord
-		if err := json.Unmarshal(data, &record); err == nil {
-			applyPlatformTemplatePromptOverrides(&record)
-			return &record, nil
+	record, found, err := cache.LoadJSONWithEmpty(ctx, r.cache, &r.group, key, templateCacheTTL, templateCacheEmptyTTL, func(loadCtx context.Context) (*TemplateRecord, bool, error) {
+		record, err := r.inner.GetTemplateByID(loadCtx, id, tenantID)
+		if err != nil {
+			if errors.Is(err, pgx.ErrNoRows) {
+				return nil, false, nil
+			}
+			return nil, false, err
 		}
-	}
-
-	record, err := r.inner.GetTemplateByID(ctx, id, tenantID)
+		return record, true, nil
+	})
 	if err != nil {
 		return nil, err
 	}
-
-	if data, err := json.Marshal(record); err == nil {
-		_ = r.cache.Set(ctx, key, data, templateCacheTTL)
+	if !found || record == nil {
+		return nil, pgx.ErrNoRows
 	}
 	applyPlatformTemplatePromptOverrides(record)
 	return record, nil