Fix/event pagination tx watch ttl

[joel-metal]

closes #387
closes #388

fix: paginate event fetch and bound watched_txs with TTL/cap

Problem 1 — Event sync misses events on busy ledgers

fetch_events_since used a single RPC call with limit:100. On ledgers
with >100 contract events the excess events were silently dropped,
causing the sync cursor to advance past un-processed data.

Fix:

• Introduce EVENTS_PAGE_SIZE = 100 constant.
• Replace the single call with a pagination loop:
  · First iteration uses startLedger = from_ledger.
  · Subsequent iterations use the opaque cursor returned by the RPC.
  · Loop exits when page length < EVENTS_PAGE_SIZE (last page reached)
  or when the RPC returns no cursor.
• RPC errors on any page still fall back to empty + rpc_errors_total
  increment, preserving the existing resilience behaviour.

Problem 2 — watched_txs grows without bound for never-finalized hashes

watched_txs was a HashSet with no eviction policy. Hashes for
transactions that never reach a terminal status (network drop, reorg,
stuck mempool) accumulated indefinitely, causing unbounded memory growth.

Fix:

• Replace HashSet with HashMap<String, Instant> to record the
  insertion timestamp of each hash.
• Add MAX_WATCHED_TXS = 1_000 hard cap:
  · watch_transaction checks the cap before inserting.
  · When at capacity the entry with the oldest Instant is evicted first.
• Add WATCHED_TX_TTL = 1 hour soft TTL:
  · run_transaction_monitor calls retain() each tick.
  · Any entry whose age exceeds WATCHED_TX_TTL is removed.
• Both eviction paths:
  · Emit tracing::warn! with the evicted hash and reason.
  · Increment cache_invalidations_total{scope="tx_watch_eviction"} so
  operators can alert on unexpected eviction rates.

Tests added

Pagination:

• test_pagination_accumulates_more_than_100_events — 250 events parsed
• test_pagination_stops_on_partial_page — last-page guard
• test_pagination_cursor_advances_without_duplicates — no cross-page dups

TTL eviction:

• test_ttl_eviction_removes_expired_entries — expired entry gone
• test_ttl_eviction_keeps_all_fresh_entries — fresh entries kept

Cap / memory growth:

• test_cap_evicts_oldest_when_full — size stays at MAX
• test_memory_growth_bounded_under_burst — 3×MAX insertions
  never exceed cap

Existing tx-monitor tests updated to use the new HashMap<String, Instant>
API (contains_key / insert with Instant instead of HashSet::contains /
insert).

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