P4.1: Extract reusable dependency check helper, deduplicate milestone_actions.py

- New app/services/dependency_check.py with check_milestone_deps()
- Replaces 3x duplicated JSON-parse + query + filter logic
- Supports both milestone and task dependency checking
- Returns structured DepCheckResult with ok/blockers/reason
- Refactored preflight and start endpoints to use shared helper

app/api/routers/milestone_actions.py

@@ -3,7 +3,6 @@
 Provides freeze / start / close actions on milestones.
 completed is triggered automatically when the sole release maintenance task finishes.
 """
-import json
 from datetime import datetime, timezone
 from typing import Optional
 
@@ -18,6 +17,7 @@ from app.models import models
 from app.models.milestone import Milestone, MilestoneStatus
 from app.models.task import Task, TaskStatus
 from app.services.activity import log_activity
+from app.services.dependency_check import check_milestone_deps
 
 router = APIRouter(
     prefix="/projects/{project_id}/milestones/{milestone_id}/actions",
@@ -101,38 +101,13 @@ def preflight_milestone_actions(
 
     # --- start pre-check (only meaningful when status == freeze) ---
     if ms_status == "freeze":
-        blockers: list[str] = []
-
-        # milestone dependencies
-        dep_ms_ids = []
-        if ms.depend_on_milestones:
-            try:
-                dep_ms_ids = json.loads(ms.depend_on_milestones)
-            except (json.JSONDecodeError, TypeError):
-                dep_ms_ids = []
-        if dep_ms_ids:
-            dep_milestones = db.query(Milestone).filter(Milestone.id.in_(dep_ms_ids)).all()
-            incomplete = [m.id for m in dep_milestones if _ms_status_value(m) != "completed"]
-            if incomplete:
-                blockers.append(f"Dependent milestones not completed: {incomplete}")
-
-        # task dependencies
-        dep_task_ids = []
-        if ms.depend_on_tasks:
-            try:
-                dep_task_ids = json.loads(ms.depend_on_tasks)
-            except (json.JSONDecodeError, TypeError):
-                dep_task_ids = []
-        if dep_task_ids:
-            dep_tasks = db.query(Task).filter(Task.id.in_(dep_task_ids)).all()
-            incomplete_tasks = [t.id for t in dep_tasks if (t.status.value if hasattr(t.status, "value") else t.status) != "completed"]
-            if incomplete_tasks:
-                blockers.append(f"Dependent tasks not completed: {incomplete_tasks}")
-
-        if blockers:
-            result["start"] = {"allowed": False, "reason": "; ".join(blockers)}
-        else:
+        dep_result = check_milestone_deps(
+            db, ms.depend_on_milestones, ms.depend_on_tasks,
+        )
+        if dep_result.ok:
             result["start"] = {"allowed": True, "reason": None}
+        else:
+            result["start"] = {"allowed": False, "reason": dep_result.reason}
 
     return result
 
@@ -232,51 +207,15 @@ def start_milestone(
             detail=f"Cannot start: milestone is '{_ms_status_value(ms)}', expected 'freeze'",
         )
 
-    # Dependency check — milestone dependencies
-    dep_ms_ids = []
-    if ms.depend_on_milestones:
-        try:
-            dep_ms_ids = json.loads(ms.depend_on_milestones)
-        except (json.JSONDecodeError, TypeError):
-            dep_ms_ids = []
-
-    if dep_ms_ids:
-        dep_milestones = (
-            db.query(Milestone)
-            .filter(Milestone.id.in_(dep_ms_ids))
-            .all()
+    # Dependency check (P4.1 — shared helper)
+    dep_result = check_milestone_deps(
+        db, ms.depend_on_milestones, ms.depend_on_tasks,
+    )
+    if not dep_result.ok:
+        raise HTTPException(
+            status_code=400,
+            detail=f"Cannot start: {dep_result.reason}",
         )
-        incomplete = [
-            m.id
-            for m in dep_milestones
-            if _ms_status_value(m) != "completed"
-        ]
-        if incomplete:
-            raise HTTPException(
-                status_code=400,
-                detail=f"Cannot start: dependent milestones not completed: {incomplete}",
-            )
-
-    # Dependency check — task dependencies
-    dep_task_ids = []
-    if ms.depend_on_tasks:
-        try:
-            dep_task_ids = json.loads(ms.depend_on_tasks)
-        except (json.JSONDecodeError, TypeError):
-            dep_task_ids = []
-
-    if dep_task_ids:
-        dep_tasks = db.query(Task).filter(Task.id.in_(dep_task_ids)).all()
-        incomplete_tasks = [
-            t.id
-            for t in dep_tasks
-            if (t.status.value if hasattr(t.status, "value") else t.status) != "completed"
-        ]
-        if incomplete_tasks:
-            raise HTTPException(
-                status_code=400,
-                detail=f"Cannot start: dependent tasks not completed: {incomplete_tasks}",
-            )
 
     ms.status = MilestoneStatus.UNDERGOING
     ms.started_at = datetime.now(timezone.utc)