Document vault lifecycle states in `backend/docs/vault-states.md`

[Jayy4rl]

Context: The vault can be in multiple states (Funding, Active, Matured, Cancelled, Emergency). Developers integrating with the API need to understand valid transitions and their implications.

Requirements:

• Create backend/docs/vault-states.md covering: all states, the transitions between them (as a table or diagram), which API operations are allowed per state, and which webhook events are fired on each transition.
• Keep under 80 lines.

Acceptance criteria:

• A developer can determine from the document alone whether a deposit is allowed in a given state.