fix(db): avoid long flush stall on restart

[EddieHouston]

Summary

Fix freeze that can occur on restart of a mature electrs DB built from 91b883a or later,
and default to steady-state compaction triggers so restarts after initial sync get low read
amplification immediately.

enable_auto_compaction() was tightening level0_stop_writes_trigger from the bulk-load
value (512) to the RocksDB default (36) on every update() call, not just once. DB::open
applied bulk-load triggers at startup, so after any restart L0 can legitimately hold more
than 36 files. The reset instantly put the DB into pre-flush stall territory, and the
end-of-batch db.flush() parked inside WaitUntilFlushWouldNotStallWrites until background
compaction brought L0 below 36.

In testnet this caused over 1 hour of indexer freeze on restart.

Fix

• DB::open() now defaults to steady-state triggers (RocksDB defaults: 4/20/36). After
  opening, it checks the F sentinel key and calls apply_bulk_load_triggers() only when
  F is absent (initial sync not yet complete). On normal restarts (F present), the DB
  stays on tight triggers from the start.

• apply_bulk_load_triggers() — new method that widens L0 triggers (64/256/512) and
  disables pending-compaction-bytes stalls for initial sync.

• apply_steady_state_triggers() — restores RocksDB defaults after full_compaction()
  drains L0. Called once per DB lifetime inside the F sentinel gate.

• enable_auto_compaction() — minimal flag set (disable_auto_compactions=false), safe
  to call on every update().

• Operator logging — DB::open and start_auto_compactions now log at info level which
  trigger profile is active and whether full compaction runs or is skipped.

Test plan

• cargo check clean
• cargo test --lib — 8/8 pass, including new_index::db::tests::*
• cargo test --test electrum — 4/4 pass
• cargo test --test rest — 22/22 pass
• Deploy to testnet and verify restart completes flush in under a second
• Confirm Manual flush start → Manual flush finished in RocksDB LOG are within ms
• Verify synced tip resumes advancing from ZMQ notifications immediately after restart