[E4-1] Set and verify base rate = 0 on the dedicated sequencer

[salindne]

Set maxBaseTrafficAmount = 0 so availableTraffic = extraTrafficPurchased - extraTrafficConsumed (no free allowance). Verify end-to-end (onboarding + rate-limiter paths) on a running dedicated sync.

Key files: Amulet baseRateTrafficLimits (DecentralizedSynchronizer.daml:30-37); deploy config baseRateBurstWindowMins

Acceptance criteria:

• With base rate 0, a member with no purchase cannot transact; after a grant it can, drawing down
• Onboarding still works with base rate 0 (confirm no breakage)

Epic: #7

Source: docs/planning/extending-mainnet-work-plan.md