refactor: unify active assignment checks and add project-type guards

- Replace all UnitAssignment "active" checks from `status == "active"` to
  `assigned_until == None` in both project_locations.py and projects.py.
  This aligns with the canonical definition: active = no end date set.
  (status field is still set in sync, but is no longer the query criterion)

- Add `_require_sound_project()` helper to both routers and call it at the
  top of every sound-monitoring-specific endpoint (FTP browser, FTP downloads,
  RND file viewer, all Excel report endpoints, combined report wizard,
  upload-all, NRL live status, NRL data upload). Vibration projects hitting
  these endpoints now receive a clear 400 instead of silently failing or
  returning empty results.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/project_locations.py

@@ -35,6 +35,19 @@ from backend.templates_config import templates
 router = APIRouter(prefix="/api/projects/{project_id}", tags=["project-locations"])
 
 
+# ============================================================================
+# Shared helpers
+# ============================================================================
+
+def _require_sound_project(project) -> None:
+    """Raise 400 if the project is not a sound_monitoring project."""
+    if not project or project.project_type_id != "sound_monitoring":
+        raise HTTPException(
+            status_code=400,
+            detail="This feature is only available for Sound Monitoring projects.",
+        )
+
+
 # ============================================================================
 # Session period helpers
 # ============================================================================
@@ -98,11 +111,11 @@ async def get_project_locations(
     # Enrich with assignment info
     locations_data = []
     for location in locations:
-        # Get active assignment
+        # Get active assignment (active = assigned_until IS NULL)
         assignment = db.query(UnitAssignment).filter(
             and_(
                 UnitAssignment.location_id == location.id,
-                UnitAssignment.status == "active",
+                UnitAssignment.assigned_until == None,
             )
         ).first()
 
@@ -258,11 +271,11 @@ async def delete_location(
     if not location:
         raise HTTPException(status_code=404, detail="Location not found")
 
-    # Check if location has active assignments
+    # Check if location has active assignments (active = assigned_until IS NULL)
     active_assignments = db.query(UnitAssignment).filter(
         and_(
             UnitAssignment.location_id == location_id,
-            UnitAssignment.status == "active",
+            UnitAssignment.assigned_until == None,
         )
     ).count()
 
@@ -569,9 +582,9 @@ async def get_available_units(
         )
     ).all()
 
-    # Filter out units that already have active assignments
+    # Filter out units that already have active assignments (active = assigned_until IS NULL)
     assigned_unit_ids = db.query(UnitAssignment.unit_id).filter(
-        UnitAssignment.status == "active"
+        UnitAssignment.assigned_until == None
     ).distinct().all()
     assigned_unit_ids = [uid[0] for uid in assigned_unit_ids]
 
@@ -747,6 +760,9 @@ async def upload_nrl_data(
     from datetime import datetime
 
     # Verify project and location exist
+    project = db.query(Project).filter_by(id=project_id).first()
+    _require_sound_project(project)
+
     location = db.query(MonitoringLocation).filter_by(
         id=location_id, project_id=project_id
     ).first()
@@ -925,15 +941,18 @@ async def get_nrl_live_status(
     Fetch cached status from SLMM for the unit assigned to this NRL and
     return a compact HTML status card. Used in the NRL overview tab for
     connected NRLs. Gracefully shows an offline message if SLMM is unreachable.
+    Sound Monitoring projects only.
     """
     import os
     import httpx
 
-    # Find the assigned unit
+    _require_sound_project(db.query(Project).filter_by(id=project_id).first())
+
+    # Find the assigned unit (active = assigned_until IS NULL)
     assignment = db.query(UnitAssignment).filter(
         and_(
             UnitAssignment.location_id == location_id,
-            UnitAssignment.status == "active",
+            UnitAssignment.assigned_until == None,
         )
     ).first()