Don't recreate Default product when config database already has products (#1386)

[mmido6039]

Fixes #1386
Also fixes #3810 (duplicate)

Problem

If 'Default.sqlite' is deleted while the config database still has
product(s) registered (the old 'Default' entry, or any other product),
the server crashes on startup:

ValueError: Called create_initial_run_database on non-empty config
database -- you shouldn't have done this!

Root cause

'create_default_product' in server_init_start() was decided purely from
whether 'Default.sqlite' exists on disk. It never checked whether the
config database itself already had products. Deleting the sqlite file
alone was enough to re-trigger 'add_initial_run_database()', which
explicitly refuses to run on a non-empty config database.

Fix

Added 'config_db_has_products()', which queries the config database for
any existing product. 'create_default_product' now also requires the
config database to be empty, not just the sqlite file to be missing.
First-time setup (empty DB) is unaffected; an existing-but-file-missing
product now falls through to the normal per-product status/upgrade
handling instead of crashing.

Testing

Added web/server/tests/unit/test_config_db_has_products.py (in-memory
SQLite, real ORM models):

• empty config DB reports no products (first-time setup still works)
• a config DB with an existing 'Default' product (the exact bug
  scenario) is correctly detected as non-empty
• any other registered product also correctly prevents auto-creation

All tests pass; pycodestyle clean.

[bruntib]

When I start the sever with a deleted PostgreSQL product database, then server start still fails with some error message. However, that's a different kind, so that could be fixed in a different PR.