Refactor: reduce code duplication with shared abstractions

- Add BaseClient HTTP abstraction (internal/api/http.go) to eliminate
  duplicated HTTP boilerplate across Todoist, Trello, and PlanToEat clients
- Add response helpers (internal/handlers/response.go) for JSON/HTML responses
- Add generic cache wrapper (internal/handlers/cache.go) using Go generics
- Consolidate HandleCompleteAtom/HandleUncompleteAtom into handleAtomToggle
- Merge TabsHandler into Handler, delete tabs.go
- Extract sortTasksByUrgency and filterAndSortTrelloTasks helpers
- Update tests to work with new BaseClient structure

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

1 files changed, 52 insertions, 0 deletions

diff --git a/internal/handlers/cache.go b/internal/handlers/cache.go
new file mode 100644
index 0000000..a2f534e
--- /dev/null
+++ b/internal/handlers/cache.go
@@ -0,0 +1,52 @@
+package handlers
+
+import (
+	"context"
+	"log"
+
+	"task-dashboard/internal/store"
+)
+
+// CacheFetcher defines the interface for fetching and caching data
+type CacheFetcher[T any] struct {
+	Store        *store.Store
+	CacheKey     string
+	TTLMinutes   int
+	Fetch        func(ctx context.Context) ([]T, error)
+	GetFromCache func() ([]T, error)
+	SaveToCache  func([]T) error
+}
+
+// FetchWithCache fetches data from cache or API with fallback
+func (cf *CacheFetcher[T]) FetchWithCache(ctx context.Context, forceRefresh bool) ([]T, error) {
+	// Check cache validity
+	if !forceRefresh {
+		valid, err := cf.Store.IsCacheValid(cf.CacheKey)
+		if err == nil && valid {
+			return cf.GetFromCache()
+		}
+	}
+
+	// Fetch from API
+	data, err := cf.Fetch(ctx)
+	if err != nil {
+		// Try to return cached data even if stale
+		cachedData, cacheErr := cf.GetFromCache()
+		if cacheErr == nil && len(cachedData) > 0 {
+			return cachedData, nil
+		}
+		return nil, err
+	}
+
+	// Save to cache
+	if err := cf.SaveToCache(data); err != nil {
+		log.Printf("Failed to save to cache (%s): %v", cf.CacheKey, err)
+	}
+
+	// Update cache metadata
+	if err := cf.Store.UpdateCacheMetadata(cf.CacheKey, cf.TTLMinutes); err != nil {
+		log.Printf("Failed to update cache metadata (%s): %v", cf.CacheKey, err)
+	}
+
+	return data, nil
+}