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/postgresql-dialect.ts

@@ -0,0 +1,81 @@
+/**
+ * PostgreSQL Dialect Implementation
+ *
+ * Encapsulates PostgreSQL-specific SQL syntax for:
+ * - Table name quoting (double-quoted "schema"."table")
+ * - Positional parameter placeholders ($1, $2, ...) — 1-based
+ * - INSERT ... ON CONFLICT ... DO UPDATE SET upsert
+ * - LIMIT/OFFSET pagination
+ */
+
+import type { SqlDialect } from '@types/sql-dialect.types'
+
+export class PostgreSqlDialect implements SqlDialect {
+  readonly dbType = 'postgresql' as const
+
+  quoteTableName(schema: string, table: string): string {
+    return `"${schema}"."${table}"`
+  }
+
+  param(index: number): string {
+    return `$${index + 1}`
+  }
+
+  params(count: number): string {
+    return Array.from({ length: count }, (_, i) => `$${i + 1}`).join(',')
+  }
+
+  currentTimestamp(): string {
+    return 'CURRENT_TIMESTAMP'
+  }
+
+  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((_, i) => `$${startParamIndex + i + 1}`)
+      .join(', ')
+
+    const conflictKeys = keyColumns.map((col) => `"${col}"`).join(', ')
+
+    const nonKeyColumns = allColumns.filter((col) => !keyColumns.includes(col))
+    const updateSet = nonKeyColumns
+      .map((col) => `"${col}" = EXCLUDED."${col}"`)
+      .join(', ')
+
+    const sql = [
+      `INSERT INTO ${table} (${columns}) VALUES (${placeholders})`,
+      `ON CONFLICT (${conflictKeys})`,
+      `DO UPDATE SET ${updateSet}`
+    ].join(' ')
+
+    return {
+      sql,
+      nextParamIndex: startParamIndex + allColumns.length
+    }
+  }
+
+  paginate(params: {
+    sql: string
+    limit: number
+    offset?: number
+    paramIndex: number
+  }): { sql: string; nextParamIndex: number } {
+    const { sql, limit, offset, paramIndex } = params
+
+    return {
+      sql: `${sql} LIMIT ${limit} OFFSET ${offset ?? 0}`,
+      nextParamIndex: paramIndex
+    }
+  }
+
+  maxBatchRows(_columnsPerRow: number): number {
+    return 1000
+  }
+}