BE-PR-010: deprecate feat_task_id — retain column, read-only compat

- Updated model docstring with full deprecation strategy
- Updated column comment to mark as deprecated (BE-PR-010)
- Updated schema/router comments for deprecation clarity
- Added deprecation doc: docs/BE-PR-010-feat-task-id-deprecation.md
- feat_task_id superseded by Task.source_proposal_id (BE-PR-008)

docs/BE-PR-010-feat-task-id-deprecation.md

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+# BE-PR-010: `feat_task_id` Deprecation & Compatibility Strategy
+
+> Date: 2026-03-30
+
+## Background
+
+The `feat_task_id` column on the `proposes` table was used by the **old** Proposal
+Accept flow to store the ID of the single `story/feature` task generated when a
+Proposal was accepted.
+
+With the new Essential-based Accept flow (BE-PR-007 / BE-PR-008), accepting a
+Proposal now generates **multiple** story tasks (one per Essential), tracked via:
+- `Task.source_proposal_id` → FK back to the Proposal
+- `Task.source_essential_id` → FK back to the specific Essential
+
+This makes `feat_task_id` obsolete.
+
+## Decision: Retain Column, Deprecate Semantics
+
+| Aspect | Decision |
+|--------|----------|
+| DB column | **Retained** — no schema migration required now |
+| Existing data | Legacy rows with a non-NULL `feat_task_id` continue to expose the value via API |
+| New writes | **Prohibited** — new accept flow does NOT write `feat_task_id` |
+| API response | Field still present in `ProposalResponse` for backward compat |
+| Client guidance | Use `generated_tasks` on the Proposal detail endpoint instead |
+| Future removal | Column will be dropped in a future migration once all clients have migrated |
+
+## Read Compatibility
+
+- `GET /projects/{id}/proposals` — returns `feat_task_id` (may be `null`)
+- `GET /projects/{id}/proposals/{id}` — returns `feat_task_id` + `generated_tasks[]`
+- `PATCH /projects/{id}/proposals/{id}` — `feat_task_id` in request body is silently ignored
+
+## Migration Path for Clients
+
+### Backend consumers
+Use `Proposal.generated_tasks` relationship (or query `Task` by `source_proposal_id`).
+
+### Frontend
+Replace `propose.feat_task_id` references with the `generated_tasks` array from
+the detail endpoint. The detail page should list all generated tasks, not just one.
+
+### CLI
+CLI does not reference `feat_task_id`. No changes needed.
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `app/models/proposal.py` | Updated docstring & column comment with deprecation notice |
+| `app/schemas/schemas.py` | Marked `feat_task_id` field as deprecated |
+| `app/api/routers/proposals.py` | Updated comments; field still serialized read-only |
+| `tests/test_propose.py` | Updated accept tests to assert `feat_task_id is None` |
+
+## Frontend References (to be updated in FE-PR-002+)
+
+- `src/types/index.ts:139` — `feat_task_id: string | null`
+- `src/pages/ProposeDetailPage.tsx:145,180-181` — displays feat_task_id
+- `src/pages/ProposesPage.tsx:83` — displays feat_task_id in list
+
+These will be addressed when the frontend Proposal/Essential tasks are implemented.