perf(server): scope paused-goal task lookup by goalId

[kvnloo]

Problem

Pausing a goal currently loads every durable task for the owner, then filters by goalId in JavaScript. Terminal task history is retained, so this path gets slower as the workspace ages.

A downstream PGlite benchmark with 2,000 ~2KB task rows measured roughly:

• full task list + JS filter: ~236ms / ~3.0MB
• SQL goalId filter: ~6.9ms / ~78KB

Change

Add Store.listByGoalId() with the same updated_at DESC,id ordering as list(), and use it only for the paused-goal cascade.

The remaining predicates are unchanged:

• terminal tasks are still ignored;
• already-paused tasks are still ignored;
• records with no goalId never match.

Regression

Persistence coverage compares the scoped query directly with the previous list().filter(item.goalId === id) semantics and checks owner/kind isolation.

No goal lifecycle or task-control behavior changes.