feat: production polish — recursive CTE, UNION/INTERSECT/EXCEPT, DROP INDEX, JSON path, FTS SQL, covering index, SCRAM auth

- Recursive CTE execution (WITH RECURSIVE + UNION ALL, work-table loop)
- UNION / INTERSECT / EXCEPT parser + executor (nkSetOp)
- DROP INDEX parser + executor
- VIEW DDL persistence via AST-to-SQL serializer
- JSON path operators -> and ->> (lexer, parser, IR, executor)
- FTS SQL wiring WHERE col @@ 'query' (case-insensitive term match)
- Multi-column index range scans (prefix equality + range on last col)
- Covering index optimization (skip LSM read when index covers SELECT)
- SCRAM-SHA-256 authentication (password-based validation)
- 279 tests, 0 failures

src/barabadb/query/ast.nim

@@ -53,6 +53,7 @@ type
     # Expressions
     nkBinOp
     nkUnaryOp
+    nkJsonPath      # column->'key' or column->>'key'
     nkFuncCall
     nkTypeCast
     nkPath
@@ -87,6 +88,9 @@ type
     nkVectorSimilar
     nkVectorNearest
 
+    # Set operations
+    nkSetOp
+
     # Join
     nkJoin
 
@@ -124,6 +128,9 @@ type
     bkCoalesce = "??"
     bkAssign = ":="
     bkArrow = "=>"
+    bkJsonPath = "->"
+    bkJsonPathText = "->>"
+    bkFtsMatch = "@@"
 
   UnaryOpKind* = enum
     ukNeg = "-"
@@ -138,6 +145,11 @@ type
     jkFull
     jkCross
 
+  SetOpKind* = enum
+    sdkUnion
+    sdkIntersect
+    sdkExcept
+
   SortDir* = enum
     sdAsc
     sdDesc
@@ -318,6 +330,10 @@ type
     of nkUnaryOp:
       unOp*: UnaryOpKind
       unOperand*: Node
+    of nkJsonPath:
+      jpLeft*: Node
+      jpKey*: string
+      jpAsText*: bool   # true for ->>, false for ->
     of nkFuncCall:
       funcName*: string
       funcArgs*: seq[Node]
@@ -412,6 +428,11 @@ type
       joinTarget*: Node
       joinOn*: Node
       joinAlias*: string
+    of nkSetOp:
+      setOpKind*: SetOpKind
+      setOpAll*: bool
+      setOpLeft*: Node
+      setOpRight*: Node
     of nkStar:
       discard
     of nkPlaceholder: