feat: support subscription tiers per functional node type

[tim-hm]

Goal

Expand the subscription payment system to support distinct, configurable tiers for each functional node type. This will allow us to offer varied functional-based pricing per node type, eg, a "large" and "small" tier for ⁠nildb.

Proposed solution

• The subscription configuration should support a structure like ⁠(node_type, tier_name, price). For example: ⁠('nildb', 'large', 1000).
• The configuration should be db-driven not env driven — meaning that tweaks to subscriptions and tiers doesn't require node redeployments
• The subscription cost endpoint should list all tiers and their prices.
• When a builder subscribes to a specific tier, this information must be encoded into the NUC's meta field, eg: ⁠"meta": { "tier": "large" }. This will enable downstream services like ⁠nildb to read and enforce tier limits accordingly.

Acceptance criteria

• The subscription system can be configured with different tiers per node type.
• When a user's NUC is generated, their subscribed tier is correctly embedded in the NUC's ⁠meta field.
 • The system can handle users with no tier (e.g., a default or free tier).