Act on the wj architectural review: races, limits, pool and contract gaps

Findings F2, F4, F5, F6, F7, F8, F9, F10, F11 and F13 from
wj-architectural-review.md. F1/F1a (the non-sargable LOWER pattern) and F3
(the warranty sync holding a pooled connection) are being handled separately
and are untouched here.

- A collector check-in that raced another for the same PC lost everything:
  the unique index on the application fingerprint raised, nothing caught it,
  and the whole check-in rolled back, discarding state, events and hardware,
  answering the agent with form copy meant for a person. The insert now runs
  in a savepoint, reconciles what the other transaction already wrote, one row
  per savepoint so a second collision skips a row rather than the batch, and
  the check-in commits with a warning. This is the shape the delete side of
  the same function already had.
- Enrollment is deliberately unauthenticated, so it now carries a per-address
  rate limit and a cap on pending requests, and the cap is a setting an admin
  can see and change. A flood can no longer bury the approval queue or grow
  the table without bound.
- The fleet alert poll no longer formats debug strings on every request.
- BusinessUnit is on the contract surface (0.34.0), so the endpoints plugin
  stops reaching into a core table with raw SQL.
- A test walks every asset type descriptor and proves the endpoint it names
  really refuses a caller without the declared permission.
- The plugin settings routes now need a permission; they were telling anyone
  which modules a site runs and where its components live.
- The connection pool is sized explicitly with a short timeout, and the
  arithmetic against the server's connection ceiling is written next to the
  worker count instead of being left to defaults nobody chose.
- Fleet alerts carry both the correct lowercase keys and the old camelCase
  pair. Trays older than 1.7.0 match keys case-sensitively and would have gone
  quiet on every PC not yet upgraded; the old pair goes when they are.
- The dev server proxy points at localhost again, with environment variables
  for the remote-box workflow, and one developer's address is out of the
  server certificate page.

tests/test_plugins/test_endpoints_credentials.py

@@ -688,7 +688,8 @@ def test_enrollment_listing_and_settings(client, db, app, keyheaders,
     assert response.status_code == 200, response.get_data(as_text=True)
     body = response.get_json()
     data = body['data']
-    assert data['settings'] == {'sharedkeyallowed': True, 'autoenroll': True}
+    assert data['settings'] == {'sharedkeyallowed': True, 'autoenroll': True,
+                                'maxpendingenrollments': 500}
     assert data['counts'] == {'enrolled': 1, 'notenrolled': 1,
                               'pendingcomputers': 1, 'pending': 1}
     states = {row['hostname']: row['state'] for row in data['computers']}
@@ -709,7 +710,8 @@ def test_enrollment_listing_and_settings(client, db, app, keyheaders,
                      json={'sharedkeyallowed': False}, headers=auth_headers)
     assert put.status_code == 200
     assert put.get_json()['data'] == {'sharedkeyallowed': False,
-                                      'autoenroll': True}
+                                      'autoenroll': True,
+                                      'maxpendingenrollments': 500}
     bad = client.put('/api/endpoints/enrollment/settings',
                      json={'autoenroll': 'yes'}, headers=auth_headers)
     assert bad.status_code == 400