refactor: move inactive stream records to temporary storage

[ScriptedBro]

Summary

This PR optimizes stream ledger storage by keeping active subscription records in Persistent storage while moving inactive (depleted) records to Temporary storage.

What Changed

• Added stream storage routing helpers:
  • stream_exists
  • get_stream
  • set_stream
  • remove_stream
• Updated stream lifecycle logic to read/write via the helpers instead of directly using persistent storage.
• Implemented storage policy:
  • active streams (balance > 0) -> Persistent storage
  • inactive streams (balance == 0) -> Temporary storage
• Applied this behavior across subscribe, collect, top-up, cancel, group flows, pause/unpause handling, tier migration, and batch withdrawal paths.
• Added tests to verify storage transitions:
  • inactive stream moves to temporary storage
  • top-up reactivates stream back to persistent storage
• Updated snapshots from test execution.

Why

Using Temporary storage for inactive subscription records reduces long-term ledger fee pressure while preserving active-stream durability in Persistent storage.

Validation

• Ran cargo test
• Result: 13 passed, 0 failed

Checklist

• Active streams remain in Persistent storage
• Inactive streams are moved to Temporary storage
• Added tests for inactive/active storage transitions
• Verified all existing tests still pass

Issue

Closes #17
Closes: #17

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