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- feat_task_id superseded by Task.source_proposal_id (BE-PR-008)

app/models/proposal.py

@@ -24,11 +24,21 @@ class Proposal(Base):
                           one Project.
     - ``created_by_id`` — FK to ``users.id``; the user who authored the Proposal.
                           Nullable for legacy rows created before tracking was added.
-    - ``feat_task_id``  — **DEPRECATED**.  Previously stored the single generated
-                          ``story/feature`` task id on accept.  Will be replaced by
-                          the Essential → story-task mapping (see BE-PR-008).
-                          Kept in the DB column for read-only backward compat; new
-                          code MUST NOT write to this field.
+    - ``feat_task_id``  — **DEPRECATED (BE-PR-010)**.  Previously stored the single
+                          generated ``story/feature`` task id on old-style accept.
+                          Superseded by the Essential → story-task mapping via
+                          ``Task.source_proposal_id`` / ``Task.source_essential_id``
+                          (see BE-PR-008).
+
+                          **Compat strategy:**
+                          - DB column is RETAINED for read-only backward compatibility.
+                          - Existing rows that have a value will continue to expose it
+                            via API responses (read-only).
+                          - New code MUST NOT write to this field.
+                          - Clients SHOULD migrate to ``generated_tasks`` on the
+                            Proposal detail endpoint.
+                          - Column will be dropped in a future migration once all
+                            clients have migrated.
     """
 
     __tablename__ = "proposes"  # keep DB table name for compat
@@ -60,11 +70,14 @@ class Proposal(Base):
         comment="Author of the proposal (nullable for legacy rows)",
     )
 
-    # DEPRECATED — see class docstring.  Read-only; will be removed once
-    # Essential-based accept (BE-PR-007 / BE-PR-008) is fully rolled out.
+    # DEPRECATED (BE-PR-010) — see class docstring for full compat strategy.
+    # Read-only; column retained for backward compat with legacy rows.
+    # New accept flow writes Task.source_proposal_id instead.
+    # Will be dropped in a future schema migration.
     feat_task_id = Column(
         String(64), nullable=True,
-        comment="DEPRECATED: id of the single story/feature task generated on old-style accept",
+        comment="DEPRECATED (BE-PR-010): legacy single story/feature task id. "
+                "Superseded by Task.source_proposal_id. Read-only; do not write.",
     )
 
     created_at = Column(DateTime(timezone=True), server_default=func.now())