feat(contracts): add storage TTL bump calls to prevent expired persistent & instance entries (closes #7)

[Vyacheslav-Tomashevskiy]

What

Adds Soroban storage TTL bump calls across all five contracts so contribution history, vote maps and every contract's instance state are not silently evicted by the network. Closes #7.

Why

Persistent and instance storage entries in Soroban carry a time-to-live (TTL) and are deleted once it elapses unless explicitly extended. Today no extend_ttl calls exist, so on an inactive group the TreasuryContract contribution history and VotingContract vote maps — plus every contract's admin/config — can disappear.

Changes

• bump_instance() helper in all 5 contracts (treasury, voting, loan, governance, dividend), called at the start of every state-changing entrypoint. Read-only getters intentionally do not bump — that keeps them cheap, read-only calls and avoids turning a dashboard read into a state write; any mutating call keeps instance storage alive, so an active group never loses config.
• extend_ttl on persistent storage after every write:
  • treasury::contribute → per-member Contributions(member) history
  • voting::create_proposal and voting::vote → per-proposal Votes(id) map
• Documented ledger counts (code comments). Stellar closes a ledger ~every 5s, so 1 day ≈ 17_280 ledgers:
  • Instance config: ~30 days (INSTANCE_BUMP_LEDGERS)
  • Persistent history / vote maps: ~90 days (PERSISTENT_BUMP_LEDGERS)
  • Threshold is set one day below the target, so a bump only pays rent when the entry is within a day of expiry rather than on every call.
• dividend: added missing String to the imports. The crate did not compile before this PR (error[E0425]: cannot find type String), which blocked cargo test on the whole workspace; the fix was required to add bump_instance there and to satisfy the "all existing tests pass" criterion.

Acceptance criteria

• extend_ttl called after every persistent storage write (treasury.contribute, voting.create_proposal, voting.vote)
• Instance storage bumped at the start of every mutating #[contractimpl] method (getters excluded by design, documented)
• Chosen ledger counts documented in comments
• All existing tests still pass — cargo test --workspace: 21 passed, 0 failed

/claim #7