Add on-chain metadata storage field to stream struct for arbitrary annotations

[Chuks-coderr]

Description

Extend the stream struct with an optional metadata field that stores a URI or short string so integrations can associate off-chain context such as invoice references or payment descriptions with a stream.

Acceptance Criteria

• Feature is fully implemented
• Tests pass
• Lint/type checks pass
• PR references this issue number

Notes

⚠️ Do not begin coding until you have been assigned by the maintainer.
Branch as feat/402-contract-stream-metadata where N is this issue number.