perf: reduce dots() N+1 queries to a single batch fetch

[dhruv1220]

Closes #146.

What changed

WorkspaceStore.dots() issued one SELECT * FROM dots followed by one SELECT spaceId FROM dot_spaces WHERE dotId=? per dot — N+1 database round-trips just to list the workspace. It now fetches all dot_spaces rows once and groups them in memory with a Map, cutting the listing down to 2 queries total.

Notes

• Per-dot grant ordering is preserved (grants are sorted in memory, matching the previous ORDER BY spaceId).
• A dot with no grants reads back spaceIds: [] — the grouping keys off the grants table, so a missing entry is an empty list rather than a fallback to a nonexistent column.

Testing

• New regression test in tests/workspace.test.ts: seeds dots across multiple spaces (including one with zero grants), asserts the returned spaceIds are correct, and asserts dots() issues exactly 2 prepare calls (red-checked: fails on unpatched code, which issues N+1).
• Full suite: 343 passed; the only 2 failures are the pre-existing network-dependent tests/transport.test.ts cases, which fail identically on clean main.
• tsc --noEmit, eslint, prettier --check clean; production build (npm run build) green.