feat(db): implement SqlDialect with MySQL, SQL Server, PostgreSQL dialects

Add three SqlDialect implementations with a factory function:
- MySqlDialect: positional ?, ON DUPLICATE KEY UPDATE, LIMIT/OFFSET
- SqlServerDialect: @pN params, MERGE USING, OFFSET/FETCH
- PostgreSqlDialect: $N (1-based), ON CONFLICT DO UPDATE, LIMIT/OFFSET

TDD approach: 43 tests written first, all passing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/services/database/dialects/sqlserver-dialect.ts

@@ -0,0 +1,95 @@
+/**
+ * SQL Server Dialect Implementation
+ *
+ * Encapsulates SQL Server-specific SQL syntax for:
+ * - Table name quoting (bracket notation [schema].[table])
+ * - Named parameter placeholders (@p0, @p1, ...)
+ * - MERGE ... USING upsert
+ * - OFFSET/FETCH pagination
+ */
+
+import type { SqlDialect } from '@types/sql-dialect.types'
+
+export class SqlServerDialect implements SqlDialect {
+  readonly dbType = 'sqlserver' as const
+
+  quoteTableName(schema: string, table: string): string {
+    return `[${schema}].[${table}]`
+  }
+
+  param(index: number): string {
+    return `@p${index}`
+  }
+
+  params(count: number): string {
+    return Array.from({ length: count }, (_, i) => `@p${i}`).join(',')
+  }
+
+  currentTimestamp(): string {
+    return 'GETDATE()'
+  }
+
+  upsert(params: {
+    table: string
+    keyColumns: string[]
+    allColumns: string[]
+    startParamIndex: number
+  }): { sql: string; nextParamIndex: number } {
+    const { table, keyColumns, allColumns, startParamIndex } = params
+
+    const valueParams = allColumns
+      .map((_, i) => `@p${startParamIndex + i}`)
+      .join(', ')
+    const sourceColumns = allColumns.join(', ')
+
+    const joinCondition = keyColumns
+      .map((col) => `target.${col} = source.${col}`)
+      .join(' AND ')
+
+    const nonKeyColumns = allColumns.filter((col) => !keyColumns.includes(col))
+    const updateSet = nonKeyColumns
+      .map((col) => `target.${col} = source.${col}`)
+      .join(', ')
+
+    const insertColumns = allColumns.join(', ')
+    const insertValues = allColumns.map((col) => `source.${col}`).join(', ')
+
+    const sql = [
+      `MERGE ${table} AS target`,
+      `USING (VALUES (${valueParams})) AS source (${sourceColumns})`,
+      `ON ${joinCondition}`,
+      `WHEN MATCHED THEN UPDATE SET ${updateSet}`,
+      `WHEN NOT MATCHED THEN INSERT (${insertColumns}) VALUES (${insertValues});`
+    ].join(' ')
+
+    return {
+      sql,
+      nextParamIndex: startParamIndex + allColumns.length
+    }
+  }
+
+  paginate(params: {
+    sql: string
+    limit: number
+    offset?: number
+    paramIndex: number
+  }): { sql: string; nextParamIndex: number } {
+    const { sql, limit, offset, paramIndex } = params
+
+    if (offset !== undefined) {
+      return {
+        sql: `${sql} OFFSET @p${paramIndex} ROWS FETCH NEXT @p${paramIndex + 1} ROWS ONLY`,
+        nextParamIndex: paramIndex + 2
+      }
+    }
+
+    return {
+      sql: `${sql} OFFSET 0 ROWS FETCH NEXT @p${paramIndex} ROWS ONLY`,
+      nextParamIndex: paramIndex + 1
+    }
+  }
+
+  maxBatchRows(columnsPerRow: number): number {
+    return Math.floor(2000 / columnsPerRow)
+  }
+}