feat(workspace): backend dashboard read model

[david-engelmann]

Summary

• Adds a typed load_dashboard_snapshot command that groups workspaces into kanban lanes (in-progress / review / done / backlog / canceled / archived) with a live isStreaming overlay per card.
• Reuses the same load_workspace_records + load_archived_workspace_records queries the sidebar already runs, so the dashboard never disagrees with the sidebar about a row's status / PR metadata / message counts.
• New workspace::dashboard module + DashboardSnapshot / DashboardLane / DashboardCard / DashboardLaneId types, mirrored on the frontend in src/lib/api.ts.
• Bridge handler invalidates the dashboardSnapshot query on WorkspaceListChanged / WorkspaceChanged / ActiveStreamsChanged so the dashboard stays in lockstep with the sidebar.

Why

Closes #482 (backend slice — UI is the next PR in this track). The issue asks for a kanban view that groups existing workspaces by status; the data already exists on the workspace row, this PR just exposes it in the shape a dashboard card grid wants.

Scope boundary

Included:

• workspace::dashboard module + build_dashboard_snapshot / assemble_snapshot (the second is a pure shaping helper so tests don't need to spin up the DB pool).
• DashboardLaneId kebab-case enum mirroring WorkspaceStatus plus an Archived lane; six lanes always returned in stable display order so the frontend renders columns in iteration order.
• DashboardCard flattens WorkspaceSummary and adds isStreaming: bool from ActiveStreams::snapshot_for_ui. Frontend never has to cross-reference listActiveStreams for the busy badge.
• load_dashboard_snapshot Tauri command + loadDashboardSnapshot TS wrapper + dashboardSnapshotQueryOptions factory.
• UI sync bridge: dashboard invalidates on the same events the sidebar listens for, plus on ActiveStreamsChanged so the streaming overlay stays live.

Deferred (intentionally out of scope):

• Dashboard UI rendering — that's the next PR in track E.
• Git change-line counts per workspace — those come from the heavy workspace_action_status path (one git subprocess per workspace). Better to fetch lazily per card on hover or per visible card.
• Project filtering server-side — every card already carries repoId, so the filter is a one-liner on the frontend.
• Drag-and-drop / status mutation / new task-management state — explicitly called out in the plan's review boundary.

Test plan

• cd src-tauri && cargo nextest run — 1158 / 1158 pass (4 new dashboard tests).
• cd src-tauri && cargo test --doc — clean.
• cd src-tauri && cargo fmt --all -- --check — clean.
• bun run lint — biome + clippy -D warnings clean.
• bun run typecheck — clean (new DashboardSnapshot / DashboardLane / DashboardCard / DashboardLaneId TS types).
• bun run test:frontend — 1113 / 1113 pass (existing tests unchanged; new query options + bridge handler additions are tree-shaken from non-dashboard surfaces).

Coverage added

New tests:

• workspace::dashboard::tests::snapshot_always_returns_all_six_lanes_in_stable_order
• workspace::dashboard::tests::workspaces_are_grouped_by_status_and_archived_split_into_own_lane
• workspace::dashboard::tests::is_streaming_overlays_active_workspaces_only
• workspace::dashboard::tests::lane_id_kebab_case_serialization_matches_workspace_status

The streaming test deliberately includes a defensive case for the rare boot race where an ActiveStreamSummary has workspace_id: None — the snapshot should treat those entries as unmatched rather than panic on .unwrap().

Follow-up

This is PR E1 in the dashboard track. E2 wires the actual UI: a route, a card grid using dashboardSnapshotQueryOptions, and a project filter chip set powered by repoId on each card.

[vercel]

@david-engelmann is attempting to deploy a commit to the Caspian's Team Team on Vercel.

A member of the Team first needs to authorize it.

[taroj1205]

@david-engelmann I feel like if this only includes the backend changes, then it shouldn't close the issue yet

[david-engelmann]

@taroj1205 meant to open as a draft in my fork, sorry about that