feat: Add pause/resume functionality to time tracking

Add pausedAt and pausedDuration columns to time_entries table.
New pause/resume endpoints with audit logging. Duration calculations
now correctly exclude paused time across start, stop, auto-stop,
and edit flows.

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

src/db/migrations/0010_add_timer_pause.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "time_entries" ADD COLUMN IF NOT EXISTS "paused_at" timestamp;
+ALTER TABLE "time_entries" ADD COLUMN IF NOT EXISTS "paused_duration" integer NOT NULL DEFAULT 0;