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/mysql-dialect.ts

@@ -0,0 +1,73 @@
+/**
+ * MySQL SQL Dialect Implementation
+ *
+ * Encapsulates MySQL-specific SQL syntax for:
+ * - Table name quoting (underscore-separated)
+ * - Positional parameter placeholders (?)
+ * - INSERT ... ON DUPLICATE KEY UPDATE upsert
+ * - LIMIT/OFFSET pagination
+ */
+
+import type { SqlDialect } from '@types/sql-dialect.types'
+
+export class MySqlDialect implements SqlDialect {
+  readonly dbType = 'mysql' as const
+
+  quoteTableName(schema: string, table: string): string {
+    return `${schema}_${table}`
+  }
+
+  param(_index: number): string {
+    return '?'
+  }
+
+  params(count: number): string {
+    return Array.from({ length: count }, () => '?').join(',')
+  }
+
+  currentTimestamp(): string {
+    return 'NOW()'
+  }
+
+  upsert(params: {
+    table: string
+    keyColumns: string[]
+    allColumns: string[]
+    startParamIndex: number
+  }): { sql: string; nextParamIndex: number } {
+    const { table, keyColumns, allColumns, startParamIndex } = params
+
+    const columns = allColumns.join(', ')
+    const placeholders = allColumns.map(() => '?').join(', ')
+
+    const nonKeyColumns = allColumns.filter((col) => !keyColumns.includes(col))
+    const updateClause = nonKeyColumns
+      .map((col) => `${col} = VALUES(${col})`)
+      .join(', ')
+
+    const sql = `INSERT INTO ${table} (${columns}) VALUES (${placeholders}) ON DUPLICATE KEY UPDATE ${updateClause}`
+
+    return {
+      sql,
+      nextParamIndex: startParamIndex + allColumns.length
+    }
+  }
+
+  paginate(params: {
+    sql: string
+    limit: number
+    offset?: number
+    paramIndex: number
+  }): { sql: string; nextParamIndex: number } {
+    const { sql, limit, offset, paramIndex } = params
+
+    return {
+      sql: `${sql} LIMIT ${limit} OFFSET ${offset ?? 0}`,
+      nextParamIndex: paramIndex
+    }
+  }
+
+  maxBatchRows(_columnsPerRow: number): number {
+    return 1000
+  }
+}