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.

.env.example

@@ -28,6 +28,19 @@ JWT_SECRET_KEY=change-this-to-another-secure-random-string
 # utf8mb4 too (see the DEPLOY wiki page). Both must be utf8mb4 to match the schema.
 DATABASE_URL=mysql+pymysql://shopdb:CHANGE_ME@db:3306/shopdb_flask?charset=utf8mb4
 
+# ---- Database connection pool (optional, defaults shown) ----
+# Per WORKER process, so the real ceiling is workers x (size + overflow). See
+# the arithmetic beside --workers in docker-compose.yml before raising either:
+# at the shipped 4 workers this is 4 x (10 + 10) = 80 against the mysql:8.4
+# default max_connections of 151. More workers needs a smaller pool here or a
+# higher --max-connections on the db service.
+# DB_POOL_TIMEOUT is the wait for a free connection. 5s fails fast and
+# legibly; the SQLAlchemy default of 30s turns pool exhaustion into what looks
+# like a dead database.
+DB_POOL_SIZE=10
+DB_MAX_OVERFLOW=10
+DB_POOL_TIMEOUT=5
+
 # ---- CORS (required, no wildcards in production) ----
 
 # Comma-separated list of explicit origins permitted to call the API.