feat: Implement comprehensive credit-based cost calculation and lifecycle management

[amalcaraz]

This PR depends on #827

Summary

Implements a complete credit-based payment system for Aleph resources,
extending the existing cost calculation framework to support credit
payments alongside hold and stream payments. Includes cost calculation,
balance validation, automated resource cleanup, and 1-day minimum runtime
protection.

Key Features

• Credit Cost Calculation: Per-hour credit consumption for all message
  types (VMs, programs, storage)
• Balance Validation: Pre-flight and runtime validation with 1-day minimum
  runtime requirement
• Automated Lifecycle Management: Cron job monitors credit balances and
  manages resource cleanup
• Reusable Framework: DRY implementation eliminates code duplication across
  handlers

Major Changes

Database & Configuration

• Migration 0036: Added cost_credit column to account_costs table
• Migration 0037: Registered credit_balance cron job for automated
  management
• Default Pricing: Added credit rates matching pay-as-you-go pricing across
  all service tiers

Cost System

• Extended cost calculations to include credit costs for all resource types
• Updated payment type detection: credit → superfluid → hold (priority
  order)
• Created reusable validate_balance_for_payment() function for all payment
  types

Message Handlers

• VM & Store Handlers: Added credit balance validation with 1-day runtime
  protection
• Pre-check Balance: Store handler now supports credit payments in both
  validation methods
• Exception Handling: New InsufficientCreditException with detailed error
  reporting

Automated Management

• Credit Balance Job: Hourly cron job monitors credit changes and manages
  resource lifecycle
• Grace Period: 24-hour deletion delay for resource recovery
• State Management: Handles PROCESSED ↔ REMOVING ↔ REMOVED transitions

Technical Implementation

• 1-Day Runtime Rule: daily_cost = hourly_cost * 24 - ensures minimum
  24-hour operation
• Payment Detection: Uses content.payment.is_credit for automatic credit
  payment detection
• Resource Recovery: Automatically restores resources when credits become
  sufficient

Files Changed

• Database models, cost services, message handlers, cron jobs, pricing
  configuration
• 2 database migrations for schema and cron job registration
• New reusable validation framework eliminates code duplication

Backward Compatibility

• All existing payment methods (hold, superfluid) unchanged
• No breaking API changes - credit system is purely additive
• Database migrations include defaults for existing records

Deployment

1. Run migrations: alembic upgrade head
2. Restart services to activate credit balance monitoring
3. Credit payments work immediately with default pricing

This provides a complete credit payment ecosystem with automated resource
management while maintaining full backward compatibility.