perf(history): write the history snapshot on change, off the UI thread

Every history refresh, address refresh, connect and lock-state check
rewrote the encrypted tx-history snapshot on the UI thread: a JSON
document of every row, its encryption, then INSERT OR REPLACE and a
prune, an estimated 40-120 ms on a 5k-transaction wallet, even when
nothing had changed.

Now a signature of what would be stored (about 1 ms on 5k rows)
decides whether to write. Confirmations count only by the thresholds
the UI reads (conflicted, pending, 1-9, 10-99, 100+); per-address scan
heights, exact confirmations and the tip drift every block and form a
separate part.

- Routine refreshes write a content change at most every 20 s (a
  deferred one is released from the update loop) and the per-block
  drift at most every 10 min. Nothing changed means no write.
- A mining-address relabel, a passphrase change, a rescan, a
  disconnect, a wallet lock and exit write whatever is not on disk yet,
  at once.
- The UI thread only copies the rows and the key into a job; a single
  background writer (newest job wins) builds, encrypts and stores it. A
  failed write is retried.
- Exit queues the final write so it overlaps the daemon stop, then
  waits up to 3 s for the writer; a disk that hangs past that keeps the
  previous snapshot.
- A loaded snapshot counts as stored, so the first identical refresh
  doesn't rewrite it. An address refresh re-keys the snapshot only when
  the address set changed.

After a hard kill the snapshot can now be up to ~20 s behind for rows
and 10 min for scan heights, exact confirmations and the tip (it was
one refresh); the first refresh replaces it either way. Lite builds,
which never read this cache, no longer write it on Refresh or a mining
relabel, and the screenshot capture mode's demo rows never reach it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CMakeLists.txt

@@ -496,6 +496,7 @@ set(APP_SOURCES
     src/wallet/lite_wallet_lifecycle_service.cpp
     src/data/wallet_state.cpp
     src/data/transaction_history_cache.cpp
+    src/data/transaction_history_store_policy.cpp
     src/ui/theme.cpp
     src/ui/theme_loader.cpp
     src/ui/explorer/explorer_block_cache.cpp
@@ -651,6 +652,7 @@ set(APP_HEADERS
     src/config/version.h
     src/data/wallet_state.h
     src/data/transaction_history_cache.h
+    src/data/transaction_history_store_policy.h
     src/ui/theme.h
     src/ui/theme_loader.h
     src/ui/explorer/explorer_block_cache.h
@@ -1207,6 +1209,7 @@ if(BUILD_TESTING)
         src/util/text_format.cpp
         src/data/wallet_state.cpp
         src/data/transaction_history_cache.cpp
+        src/data/transaction_history_store_policy.cpp
         src/data/address_book.cpp
         src/data/wallet_index.cpp
         src/daemon/lifecycle_adapters.cpp