test(api): cover the music auth endpoints, and always clean up the wrapper

The Spotify step-2 handler writes a Python wrapper script to a temp file
with the user's redirect URL embedded in its source, then executes it.
That is the most dangerous shape in the blueprint and had no tests.

The wrapper was deleted in the success/failure branch and again in the
TimeoutExpired handler. Any other failure from subprocess.run — no
interpreter, a fork failure, an interrupted call — reached neither, and
left a world-readable temp file containing the user's redirect URL on
disk. Cleanup moves to a finally block, which is what "delete this
whatever happens" should have been from the start.

The injection tests are the point of this file. Eight adversarial
redirect URLs (embedded quotes, backslashes, newlines, triple quotes, a
full `"; import os; os.system("id"); "`) are each pushed through the
endpoint and the generated wrapper is parsed with ast: it must still be
valid Python, the URL must still be a single string literal bound to
redirect_url, and no os.system call may appear anywhere in the tree.
json.dumps holds up, but nothing was checking that it does.

40 tests. Also pins that the two endpoints are not symmetrical despite
the matching names — only Spotify has a two-step flow and a wrapper; YTM
runs its script directly — so a later change does not "restore" a parity
that was never there.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NohXi78cwsAKtN1sCfxjUh

web_interface/blueprints/api_v3.py

@@ -6272,7 +6272,6 @@ sys.exit(proc.returncode)
                     timeout=120,
                     env=env
                 )
-                os.unlink(wrapper_path)
 
                 if result.returncode == 0:
                     return jsonify({
@@ -6287,9 +6286,13 @@ sys.exit(proc.returncode)
                         'output': result.stdout + result.stderr
                     }), 400
             except subprocess.TimeoutExpired:
+                return jsonify({'status': 'error', 'message': 'Authentication timed out'}), 408
+            finally:
+                # The wrapper carries the user's redirect URL, so it must not
+                # survive the request on any path — including a failure to
+                # launch, which the previous per-branch unlinks missed.
                 if os.path.exists(wrapper_path):
                     os.unlink(wrapper_path)
-                return jsonify({'status': 'error', 'message': 'Authentication timed out'}), 408
         else:
             # Step 1: Get authorization URL
             # Import the script's functions directly to get the auth URL