fix(postgres): widen git_function_blame.symbol_id to Text

[Aman-goel-04]

Summary

• Widens git_function_blame.symbol_id from VARCHAR(512) to Text, it mirrors WikiSymbol.symbol_id, which is already unbounded, but wasn't caught by [Bug] Postgres SQL Schema doesn't fit data for big projects, Phase 4 of 4 · Persistence fails #1565/0062 since that migration targeted symbol name columns at 255 chars, not a symbol id at 512
• Adds migration 0064 (Postgres-only, same shape as 0062)

Related Issues

Fixes #2162

Test Plan

• Tests pass (pytest)
• Lint passes (ruff check .)
• Web build passes (npm run build) (no frontend changes)

What was done

• Reproduced the original error locally against Postgres, confirmed the fix resolves it
• alembic upgrade head / downgrade -1 verified round-trip on a stamped pre-fix schema
• Added an integration test (tests/integration/test_postgres_git_function_blame_symbol_id.py), confirmed it fails without the fix and passes with it

Checklist

• My code follows the project's code style
• I have added tests for new functionality
• All existing tests still pass
• I have updated documentation if needed

[RaghavChamadiya]

Guarding the migration on the dialect is the right call, and the reason you gave for it is the one that matters: a batch_alter_table on SQLite would rebuild the table to change nothing, since it never enforced the length in the first place. The downgrade refusing to truncate rather than papering over it with left(col, 512) is also right. Integration tests do not run in CI, so the regression test is documentation of the repro for now, which is still worth having next to the migration.