refactor(api-v3): split the 10,469-line blueprint into a package

web_interface/blueprints/api_v3.py held 111 routes, 56 helpers and 181
functions in one module -- 9% of the core by line count and three times the
next largest file. It becomes a package of nine route modules grouped by path
segment, plus __init__.py for the shared imports, constants, Blueprint and
helpers.

Every route module decorates the SAME api_v3 Blueprint object, so endpoint
names stay api_v3.<function>, the URL map is unchanged and app.py is untouched.
Verified: 111 routes before, 111 after, byte-identical rules, endpoints and
methods, and every endpoint still on the one blueprint.

  plugins   3,867   config    1,178   starlark  692   system  619
  fonts       452   misc        398   wifi      361   display 326   backup 212
  __init__  1,787 (imports, constants, Blueprint, 56 helpers)

Two things the URL-map check could not catch, both found by running the suite:

1. PROJECT_ROOT = Path(__file__).parent.parent.parent. Moving the code one
   directory deeper made that resolve to web_interface/ instead of the project
   root. Nothing failed at import; it surfaced as ~110 tests failing with 404s
   and "installation script not found", because every path built from it was
   one level too shallow. Now parents[3], and test_api_v3_url_map.py asserts
   PROJECT_ROOT/run.py exists so the next move cannot repeat it.

2. Module-attribute patching. Tests do
   monkeypatch.setattr(api_v3_module, "_BACKUP_EXPORT_DIR", ...) and a route
   module that binds such a name by value never sees the patch. The shared code
   therefore stays in __init__.py rather than moving to a _common submodule --
   it has to live on the module the tests patch -- and the eleven names tests
   patch are read back through the package (_pkg.X) instead of bound by value.
   Those eleven were found by AST-scanning every setattr in the test tree, not
   by guessing; "time" is among them, used to drive a fake clock through the
   second-resolution credential-backup filenames.

Test changes are confined to what genuinely moved: patch targets that now name
the owning route module, imports of helpers, and six tests that scan the api_v3
source as a file and now read the package directory.

Full suite: 4,278 passed, 68 skipped, 0 failed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RRtqXDCnvnY6EQwhT5CV9

test/test_web_api.py

@@ -390,7 +390,7 @@ class TestConfigAPI:
 class TestSystemAPI:
     """Test system API endpoints."""
     
-    @patch('web_interface.blueprints.api_v3.subprocess')
+    @patch('web_interface.blueprints.api_v3.system.subprocess')
     def test_get_system_status(self, mock_subprocess, client):
         """Test getting system status."""
         # The endpoint returns 503 without psutil, which is an optional
@@ -407,7 +407,7 @@ class TestSystemAPI:
         data = json.loads(response.data)
         assert 'service' in data or 'status' in data or 'active' in data
     
-    @patch('web_interface.blueprints.api_v3.subprocess')
+    @patch('web_interface.blueprints.api_v3.system.subprocess')
     def test_get_system_version(self, mock_subprocess, client):
         """Test getting system version."""
         mock_result = MagicMock()
@@ -421,7 +421,7 @@ class TestSystemAPI:
         data = json.loads(response.data)
         assert 'version' in data.get('data', {}) or 'version' in data
     
-    @patch('web_interface.blueprints.api_v3.subprocess')
+    @patch('web_interface.blueprints.api_v3.system.subprocess')
     def test_execute_system_action(self, mock_subprocess, client):
         """Test executing system action."""
         mock_result = MagicMock()
@@ -502,7 +502,7 @@ class TestDisplayAPI:
         if response.status_code in [200, 201]:
             assert api_v3.cache_manager.set.called
     
-    @patch('web_interface.blueprints.api_v3._ensure_cache_manager')
+    @patch('web_interface.blueprints.api_v3.display._ensure_cache_manager')
     def test_stop_on_demand_display(self, mock_ensure_cache, client):
         """Test stopping on-demand display."""