feat: support OrmarConfig(comment=...) for table comments

[collerek]

Adds comment= to OrmarConfig (and OrmarConfig.copy()) so models can declare a SQL COMMENT ON TABLE value. The comment is forwarded to the underlying SQLAlchemy Table and emitted by metadata.create_all() on PostgreSQL and MySQL; SQLite ignores it.

copy() inherits the parent's comment by default — pass comment=None to clear it or a new string to override, mirroring the existing schema semantics.

Closes #1240

[codspeed-hq]

Merging this PR will degrade performance by 10.02%

❌ 1 (👁 1) regressed benchmark
✅ 97 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
👁	WallTime	test_select_related_nested_merge[5-5]	29.9 ms	33.3 ms	-10.02%

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_{Comparing feat/issue-1240-table-comment (b8886bf) with master (1271803)}