feat: caller-supplied idempotency key for place_bets batch

[Keengfk]

feat: caller-supplied idempotency key for place_bets batch

Summary

place_bets previously had no per-batch deduplication token. A client retry
after a partial failure could double-submit bets. This PR adds a
caller-supplied BytesN<32> idempotency key consumed atomically on the first
successful call.

Changes

storage.rs

• Added DataKey::PlaceBetsIdem(Address, BytesN<32>) — keyed per user so keys
  from different users never collide
• Added PLACE_BETS_IDEM_TTL_LEDGERS = 7 * LEDGERS_PER_DAY (≈ 7 days at 5
  s/ledger); documented inline

err.rs

• Added Error::IdempotentBatchAlreadyApplied = 509

bets.rs

• Added idempotency_key: BytesN<32> parameter to BetManager::place_bets
• Guard checks for existing key before any validation; rejects with typed error
• Key is written to persistent storage with TTL only after all bets succeed
  (atomic)

lib.rs

• Updated place_bets entrypoint signature to forward the new parameter

place_bets_idempotency_tests.rs (new)

• 5 tests: fresh key accepted, duplicate key rejected, different key accepted,
  key scoped per user, key reusable after TTL

Acceptance criteria

┌───────────┬────────┐
│ Criterion │ Status │
├──────────────────────────────────────────────────────────┼────────┤
│ Repeated key rejected with IdempotentBatchAlreadyApplied │ ✅ │
├──────────────────────────────────────────────────────────┼────────┤
│ TTL documented in storage.rs constants │ ✅ │
├──────────────────────────────────────────────────────────┼────────┤
│ No unwrap() introduced │ ✅ │
├──────────────────────────────────────────────────────────┼────────┤
│ Key scoped to (user, key) — no cross-user collisions │ ✅ │
└──────────────────────────────────────────────────────────┴────────┘

Testing

cargo test -p predictify-hybrid place_bets_idempotency -- --nocapture
cargo test -p predictify-hybrid batch_operations_tests -- --nocapture
closes #621

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