fix: check negative cache in Service.Get() before fetching upstream

The checkNegativeCache() method existed but was never called, making
negative caching (for failed fetches) completely non-functional.
Failed URLs were being re-fetched on every request.

Add negative cache check at the start of Service.Get() to short-circuit
requests for recently-failed URLs.

Fixes #3

internal/imgcache/negative_cache_test.go

@@ -2,27 +2,11 @@ package imgcache
 
 import (
 	"context"
-	"database/sql"
 	"errors"
 	"testing"
 	"time"
-
-	"sneak.berlin/go/pixa/internal/database"
 )
 
-func setupTestDB(t *testing.T) *sql.DB {
-	t.Helper()
-	db, err := sql.Open("sqlite", ":memory:")
-	if err != nil {
-		t.Fatal(err)
-	}
-	if err := database.ApplyMigrations(db); err != nil {
-		t.Fatal(err)
-	}
-	t.Cleanup(func() { db.Close() })
-	return db
-}
-
 func TestNegativeCache_StoreAndCheck(t *testing.T) {
 	db := setupTestDB(t)
 	dir := t.TempDir()