[Observability] Logging: common/auth (JWT, computing-unit access)

Part of #4070.

Add auth diagnostics without leaking secrets.

JWT auth outcomes and computing-unit access decisions were hard to trace; expected token failures were logged at ERROR (pageable noise).

JwtAuthFilter success breadcrumb (uid only, never the token).
JwtParser parse failures downgraded ERROR -> WARN (forged/expired tokens are expected input).
ComputingUnitAccess logs the resolved access decision.
Non-functional; no behavior change.

Branch: chore/logging-common-auth

/parent-issue #4070

/take

Closing for now — consolidating the approach/procedure into discussion #5355 for feedback before raising PRs. Will reopen as PRs land.

/unparent-issue #4070

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