Add the hinge pattern: Deadlift, RDL, Glute Bridge, Hip Thrust, Back Extension

The biggest hole in the coverage matrix - the hinge had zero entries in
any modality. Deadlift and Romanian Deadlift are the library's first
standing (camera.zoom) and first barbell motions.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Exercise Library/Back Extension/motion.json

@@ -0,0 +1,37 @@
+{
+  "name": "Back Extension",
+  "primary": 2,
+  "working": ["spine"],
+  "props": [
+    {"type": "scene", "shapes": [
+      {"kind": "line", "pts": [[102, 124], [154, 106]], "w": 9, "depth": 8},
+      {"kind": "line", "pts": [[130, 122], [130, 150]], "w": 5},
+      {"kind": "line", "pts": [[58, 142], [88, 142]], "w": 5, "depth": 6}
+    ]},
+    {"type": "roller", "at": ["foot_r", "foot_l"], "r": 4.5, "side": 1, "back": 4}
+  ],
+  "frames": [
+    {
+      "hold": 0.4, "tween": 1.0,
+      "root": {"pos": [150, 102], "pitch": 112},
+      "spine": [0, 12],
+      "neck": 10, "head": -30,
+      "shoulder_r": 30, "elbow_r": 140,
+      "shoulder_l": 30, "elbow_l": 140,
+      "hip_r": 48, "knee_r": 12, "ankle_r": 12,
+      "hip_l": 48, "knee_l": 12, "ankle_l": 12,
+      "pins": {"foot_r": [72, 136], "foot_l": [76, 138]}
+    },
+    {
+      "hold": 0.6, "tween": 1.1,
+      "root": {"pos": [150, 102], "pitch": 65},
+      "spine": [0, -6],
+      "neck": 5, "head": -55,
+      "shoulder_r": 30, "elbow_r": 140,
+      "shoulder_l": 30, "elbow_l": 140,
+      "hip_r": 2, "knee_r": 12, "ankle_r": 12,
+      "hip_l": 2, "knee_l": 12, "ankle_l": 12,
+      "pins": {"foot_r": [72, 136], "foot_l": [76, 138]}
+    }
+  ]
+}