refactor: delete the legacy session model

Two session models shared one process. `/v1/*` and `/v1/stream` are the live
one; `/api/sessions*` and `/api/compare_live_vs_still` were the earlier model,
still constructed on every `create_app()` through `build_legacy_scans` even
though nothing called them. The web client and `stream_client.py` speak only
`/v1`.

Deleted: `observe.py` (ScanService/PerceptionService/PerceptionAdapter),
`yolo_adapter.py` (imported only by the legacy builder), `pipeline.py` and
`test_suite.py` (the old single-image CLI, reachable only from each other),
`test_observe.py` (its Session* tests), the eight legacy route handlers,
`build_legacy_scans` with its two facades, `create_fastapi_app`, and the
`scans` parameter on `create_app`.

`IdentityRankingTests` moved to `test_identity.py` before `test_observe.py` was
deleted: those five ranking rules are the live path's contract, not the legacy
model's. `test_api.py` keeps every `/v1` and WebSocket test.

Verified: 246 tests OK (2 skipped), and a route dump of a freshly built app
shows no `/api/*` path with all nine `/v1` routes and the WebSocket intact.

recognization/code/test_identity.py

@@ -0,0 +1,52 @@
+"""Ranking contract for decide_identity: what a score set is allowed to name.
+
+Moved out of test_observe.py when the legacy session model was deleted; the
+ranking rules are the live path's, not the legacy one's.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+import unittest
+
+current_dir = os.path.dirname(os.path.abspath(__file__))
+if current_dir not in sys.path:
+    sys.path.insert(0, current_dir)
+
+from identity import (  # noqa: E402
+    IDENTITY_ACCEPTED,
+    IDENTITY_AMBIGUOUS,
+    IDENTITY_REJECTED,
+    IDENTITY_UNKNOWN,
+    decide_identity,
+)
+
+
+class IdentityRankingTests(unittest.TestCase):
+    def test_zero_scores_are_rejected_not_a_sku(self):
+        ranked = decide_identity({"sku_a": 0.0, "sku_b": 0.0})
+        self.assertEqual(ranked["identity_level"], IDENTITY_REJECTED)
+        self.assertIsNone(ranked["best_sku"])
+        self.assertEqual(ranked["status"], IDENTITY_REJECTED)
+
+    def test_empty_scores_are_rejected(self):
+        ranked = decide_identity({})
+        self.assertEqual(ranked["identity_level"], IDENTITY_REJECTED)
+        self.assertIsNone(ranked["best_sku"])
+
+    def test_weak_unique_score_is_unknown(self):
+        ranked = decide_identity({"sku_a": 0.4, "sku_b": 0.0})
+        self.assertEqual(ranked["identity_level"], IDENTITY_UNKNOWN)
+        self.assertIsNone(ranked["best_sku"])
+
+    def test_close_scores_are_ambiguous(self):
+        ranked = decide_identity({"sku_a": 20.0, "sku_b": 19.5})
+        self.assertEqual(ranked["identity_level"], IDENTITY_AMBIGUOUS)
+        self.assertIsNone(ranked["best_sku"])
+
+    def test_unique_strong_score_is_accepted(self):
+        ranked = decide_identity({"sku_a": 40.0, "sku_b": 5.0})
+        self.assertEqual(ranked["identity_level"], IDENTITY_ACCEPTED)
+        self.assertEqual(ranked["best_sku"], "sku_a")
+        self.assertEqual(ranked["status"], "success")