feat(shared,api): add pause + sort_order columns and contracts

Adds server-side pause accounting and activity ordering primitives.
- WorkSession contract gains paused_seconds (number) and paused_at (ISO string | null).
- Activity contract gains sort_order (number); new ReorderActivitiesInput zod.
- work_sessions += paused_seconds (int NOT NULL DEFAULT 0) + paused_at (timestamp_ms nullable).
- activities += sort_order (int NOT NULL DEFAULT 0).
- toWorkSession / toActivity map the new fields; generated migration 0003.

The new fields are additive; existing api/worker/admin tests stay green.

Products affected: SoleLog backend (apps/api), shared contracts (packages/shared)

apps/api/drizzle/0003_sharp_giant_girl.sql

@@ -0,0 +1,3 @@
+ALTER TABLE `activities` ADD `sort_order` integer DEFAULT 0 NOT NULL;--> statement-breakpoint
+ALTER TABLE `work_sessions` ADD `paused_seconds` integer DEFAULT 0 NOT NULL;--> statement-breakpoint
+ALTER TABLE `work_sessions` ADD `paused_at` integer;