fix(adapter,executor,tests): harden nimforum adapter and fix transaction/aggregate bugs

Adapter (baradb_sqlite.nim):
- getRow now returns @[] for missing rows (db_sqlite compat)
- insertID/nextId handle NULL (\N) values correctly
- preprocessQuery replaces SQLite-specific syntax:
  TIMESTAMP->DATETIME, DATETIME('now')->current timestamp, INET->STRING
- add dbBegin/dbCommit/dbRollback for transaction support

Executor:
- fix COMMIT treating empty values as DELETE (rows with only PK were lost)
- fix MIN/MAX aggregates including NULL (\N) which skewed results
- add numeric comparison for irNeq

Tests:
- stress_test: shared DB, 16 workers, correctness scan after run
- nimforum_smoke_test: 10 tests including schema, transactions, iterators

src/barabadb/query/ast.nim

@@ -428,14 +428,17 @@ type
     of nkInExpr:
       inLeft*: Node
       inRight*: Node
+      inNegated*: bool
     of nkBetweenExpr:
       betweenExpr*: Node
       betweenLow*: Node
       betweenHigh*: Node
+      betweenNegated*: bool
     of nkLikeExpr:
       likeExpr*: Node
       likePattern*: Node
       likeCaseInsensitive*: bool
+      likeNegated*: bool
     of nkIsExpr:
       isExpr*: Node
       isType*: string