feat: improved asset filter UX - collapsible filter panel, customer/location cascade, status dropdown, assigned_to filter, active filter tags, result count

Backend:
- GET /api/customers — lists all customers for filter dropdown
- GET /api/locations?customer_id=X — cascading locations per customer
- GET /api/users — lists users for assigned_to filter

Frontend:
- Collapsible 'Filters' panel with Customer→Location cascade, Status, Assigned To
- Active filter tags showing what's currently filtering, removable individually
- 'Clear all filters' button to reset all at once
- Result count display ('X assets found')
- Better search placeholder showing searchable fields
- Filter count badge on the toggle button

server.py

@@ -798,6 +798,45 @@ def health():
     return {"status": "ok"}
 
 
+# ─── Customer & Location listing (for filter dropdowns) ─────────────────────
+
+
+@app.get("/api/customers")
+def list_customers():
+    conn = get_db()
+    rows = conn.execute(
+        "SELECT id, name FROM customers ORDER BY name"
+    ).fetchall()
+    conn.close()
+    return [dict(r) for r in rows]
+
+
+@app.get("/api/locations")
+def list_locations(customer_id: Optional[int] = Query(None)):
+    conn = get_db()
+    if customer_id:
+        rows = conn.execute(
+            "SELECT id, customer_id, name FROM locations WHERE customer_id = ? ORDER BY name",
+            (customer_id,),
+        ).fetchall()
+    else:
+        rows = conn.execute(
+            "SELECT id, customer_id, name FROM locations ORDER BY customer_id, name"
+        ).fetchall()
+    conn.close()
+    return [dict(r) for r in rows]
+
+
+@app.get("/api/users")
+def list_users():
+    conn = get_db()
+    rows = conn.execute(
+        "SELECT id, username, role FROM users ORDER BY username"
+    ).fetchall()
+    conn.close()
+    return [dict(r) for r in rows]
+
+
 # ─── Task 4: POST /api/assets ──────────────────────────────────────────────