fix(admin): raise AdminError instead of sys.exit; add admin test suite

[rylinjames]

Audit §3.10 (admin A1/A2/A3/A5) — admin/ had zero tests because the library called sys.exit(2) directly, so nothing could be exercised without subprocess harnesses.

Changes

• A1 (root cause): get_admin_token / admin_request now raise AdminError(exit_code=2) instead of stderr.write + sys.exit. Each command's main() catches it, prints the message, and returns the code — CLI exit-2-on-failure behavior is preserved, but the functions are now unit-testable and usable as a library.
• A2: an unsupported HTTP method was raise ValueError inside the network try, so a programmer error got reported as "Could not reach worker". Now validated up front; the network except is narrowed to httpx.HTTPError.
• A3: warn when the admin bearer token would be sent over cleartext http:// to a non-local host (otherwise the token leaks silently).
• A5: validate --max-seats >= 1 and --limit >= 1. A4: renamed the license local that shadowed the builtin.

Tests

tests/test_admin.py — 14 cases: token presence; unsupported-method ≠ network error; 401 / 4xx / network-error wrapping; insecure-endpoint warning (and no warning for localhost); each command's main() success + AdminError exit codes + arg validation. ruff clean.

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