security(dbml): enforce identifier-to-DDL trust boundary

[seonghobae]

Security outcome

• parse PostgreSQL-compatible doubled quotes in DBML identifiers without losing Unicode, reserved words, whitespace, semicolons, or comment markers
• fail closed on NUL, empty/unterminated identifiers, more than 63 UTF-8 bytes, empty path segments, over-deep table paths, and malformed references; the HTTP route returns 422 without a partial snapshot or DDL
• centralize identifier validation and double-quote rendering for DDL export/migrations, index recommendations, Snowflake introspection, and DBML PK/FK constraint definitions
• make statement splitting quote-aware so semicolons inside identifier or string tokens cannot be mistaken for appended SQL
• add deterministic property fuzzing plus focused parser→snapshot→DDL/FK/API regressions
• document authority, failure/recovery, limitations, and APA 7 primary sources

Stack

This PR is based on #832 (exact base 1f8b859c6a174c48e1fe0608cf9db1110cb252bb) because that predecessor closes the pre-existing owned branch-coverage gaps. Review this PR's two commits after that exact base. It must not merge before #832.

Verification

• focused DBML/DDL/migration/index/Snowflake/apply suites: 59 passed
• complete backend after the predecessor: 495 passed, 1 skipped
• owned production statement/branch coverage: 100.00% (414 statements, 64 branches)
• mypy: Success: no issues found in 69 source files
• git diff --check: clean

No dependency, lockfile, database migration, frontend, or workflow changes.

Closes #747

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