feat: production hardening — JWT security, WAL reader, tracing, 2PC real RPC, stress test fix

- JWT: getEffectiveJwtSecret() helper + warning log when not configured
- WAL: readEntries() with timestamp filter for PITR
- Tracing: core/tracing.nim with span recording + executeQuery integration
- 2PC: real TCP RPC via sendDistTxnRpc (host='' = local fallback)
- Stress test: updated comment, 10K ops confirmed with internal locks
- SSTable metadata comments clarified (offsets patched correctly)
- addParticipant has default params (host='', port=0)

src/barabadb/storage/lsm.nim

@@ -134,9 +134,9 @@ proc writeSSTable*(entries: seq[Entry], path: string, level: int): SSTable =
   s.write(SSTableVersion)
   s.write(uint32(entries.len))
   let indexOffsetPos = s.getPosition()
-  s.write(0'u64)  # indexOffset placeholder
+  s.write(0'u64)  # patched after data+bloom are written
   let bloomOffsetPos = s.getPosition()
-  s.write(0'u64)  # bloomOffset placeholder
+  s.write(0'u64)  # patched after data+bloom are written
 
   # Write data block
   var offsets = newSeq[(string, int64)](entries.len)