Add Time Tracking backend API

Implementovaný kompletný backend pre time tracking:
- Nová tabuľka time_entries s foreign keys na users, projects, todos, companies
- Service layer s business logikou pre CRUD operácie
- Controller pre všetky endpointy
- Validačné schémy pomocou Zod
- Routes s autentifikáciou a validáciou
- Endpointy:
  * POST /api/time-tracking/start - Spustenie timeru
  * POST /api/time-tracking/:id/stop - Zastavenie timeru
  * GET /api/time-tracking/running - Získanie bežiaceho záznamu
  * GET /api/time-tracking/month/:year/:month - Mesačné záznamy
  * GET /api/time-tracking/stats/monthly/:year/:month - Mesačné štatistiky
  * PATCH /api/time-tracking/:id - Aktualizácia záznamu
  * DELETE /api/time-tracking/:id - Zmazanie záznamu
- Podpora pre isEdited flag pri editácii
- Kalkulácia duration v minútach

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/db/migrations/0004_add_time_entries_table.sql

@@ -0,0 +1,23 @@
+CREATE TABLE "time_entries" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"user_id" uuid NOT NULL,
+	"project_id" uuid,
+	"todo_id" uuid,
+	"company_id" uuid,
+	"start_time" timestamp NOT NULL,
+	"end_time" timestamp,
+	"duration" integer,
+	"description" text,
+	"is_running" boolean DEFAULT false NOT NULL,
+	"is_edited" boolean DEFAULT false NOT NULL,
+	"created_at" timestamp DEFAULT now() NOT NULL,
+	"updated_at" timestamp DEFAULT now() NOT NULL
+);
+--> statement-breakpoint
+ALTER TABLE "time_entries" ADD CONSTRAINT "time_entries_user_id_users_id_fk" FOREIGN KEY ("user_id") REFERENCES "public"."users"("id") ON DELETE cascade ON UPDATE no action;
+--> statement-breakpoint
+ALTER TABLE "time_entries" ADD CONSTRAINT "time_entries_project_id_projects_id_fk" FOREIGN KEY ("project_id") REFERENCES "public"."projects"("id") ON DELETE set null ON UPDATE no action;
+--> statement-breakpoint
+ALTER TABLE "time_entries" ADD CONSTRAINT "time_entries_todo_id_todos_id_fk" FOREIGN KEY ("todo_id") REFERENCES "public"."todos"("id") ON DELETE set null ON UPDATE no action;
+--> statement-breakpoint
+ALTER TABLE "time_entries" ADD CONSTRAINT "time_entries_company_id_companies_id_fk" FOREIGN KEY ("company_id") REFERENCES "public"."companies"("id") ON DELETE set null ON UPDATE no action;