feat: add multi-database dialect support

[RAprogramm]

Summary

• Add DatabaseDialect enum with support for PostgreSQL, ClickHouse, MongoDB
• Feature-gated SQL generation per database dialect
• PostgreSQL stable, ClickHouse/MongoDB as placeholders for future implementation
• Refactor sql.rs to use dialect-specific placeholders

Changes

File	Description
src/entity/parse/dialect.rs	New DatabaseDialect enum with placeholder/set_clause generation
src/entity/sql.rs	Refactored to use dialect, multi-db support structure
src/entity/parse.rs	Export DatabaseDialect, add to EntityDef
Cargo.toml	Add postgres, clickhouse, mongodb features, bump to 0.2.0

Usage

#[derive(Entity)]
#[entity(table = "users", dialect = "postgres")]
pub struct User { ... }

Test plan

• All 15 unit tests pass
• All integration tests pass
• cargo clippy clean
• cargo llvm-cov coverage check

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