feat: support multiple assignees for tasks

[pikann]

Summary

Tasks can now have multiple assignees instead of exactly one. Full-stack change spanning the schema, the Go API, the MCP server, and the web UI.

Data model

• Replaced the single tasks.assignee_id column with a task_assignees join table (task_id, member_id, assigned_at), migrated via a backfill + column drop.
• The migration is idempotent (guarded with a DO $$ ... $$ block checking information_schema.columns) since this repo's migration runner has no applied-migrations tracking table — it re-executes every .sql file on every app startup, so a plain non-idempotent backfill/drop would fail on the second startup once the source column is already gone.

Backend (services/api)

• Task.AssigneeID *uuid.UUID → AssigneeIDs []uuid.UUID across the domain, repository, service, DTO, and handler layers.
• List/filter endpoints keep their existing assignee_id / assignee_ids / assignee_id=null query params; filtering matches a task if any of its assignees is in the given set.
• CreateTask/UpdateTask accept/replace the full assignee set in one call (assignee_ids in the request body), following the same "send the full new value" convention already used for tags.
• Assignment-changed events are published once per newly added assignee, so the notification/AI-agent trigger pipeline needed no changes.
• Automation workflow status rules now replace a task's entire assignee set with the rule's configured member when they fire — the direct generalization of the previous single-assignee overwrite behavior.
• Fixed two other call sites that read assignee_id directly and would otherwise have silently broken: the WASM plugin runtime's paca.tasks_list / paca.task_get host functions, and the MCP server (apps/mcp) REST client + task formatters.

Frontend (apps/web)

• New multi-user property-field mode (chip list + checklist popover) for editing assignees in the task detail panel.
• Task cards, table rows, and subtask rows now show an overlapping avatar stack (with a +N overflow badge) and a toggle-based picker instead of a single-select popover.
• Board/list views grouped by assignee use each task's first assignee for its swimlane/column.

Docs

• Updated the DBML schema, HTTP API examples, plugin SDK reference, and AI-agent docs for assignee_ids.

Known scope boundary

MCP tools (create_task / update_task) still assign one member per call — the client translates that into a single-element assignee_ids array on the wire. Multi-assign-in-one-call for AI agents was left out of scope.

Test plan

• go build ./... && go vet ./... && go test ./... in services/api
• PACA_E2E=1 go test ./test/e2e/... — full suite, including a new TestE2ETaskManagement_MultipleAssignees (create/get/filter/partial-update/clear) and the workflow-automation e2e tests updated for replace semantics
• Manually replayed the migration twice against a fresh Postgres container to confirm idempotency (exact repro of the reported startup crash)
• bun run lint && bun run tsc -b && bun run test in apps/web — 281 tests passing
• bun run lint && npx tsc --noEmit && bun run test in apps/mcp — 605 tests passing

[pullfrog]

Important

Assignee grouping in board/table views is inconsistent with the new multi-assignee model: a task only appears under its first assignee's column, and dragging a task onto an assignee column replaces the entire assignee set. Decide the intended board behavior before merging.

Reviewed changes — migration from single assignee_id to a task_assignees join table across the API, service layer, automations, plugins, MCP client, and web UI.

• Migrated tasks.assignee_id to a task_assignees join table with backfill and idempotent DDL.
• Replaced single-value assignee fields with []uuid.UUID arrays in domain models, DTOs, and responses.
• Updated task filtering, workflow automation, and notification events to work with the new array shape.
• Added a multi-user editor and avatar stacks for selecting/displaying multiple assignees.

ℹ️ Nitpicks

• apps/e2e/tests/projects/interaction-views.spec.ts:88 still types createTask with assignee_id?: string; the API now expects assignee_ids, so the helper payload type should be updated to match even though no current caller uses it.

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using Kimi K2 (free via Pullfrog for OSS) ｜ 𝕏}