Audit remaining f-string/interpolated SQL and add CI guard

Summary

#3445 (parameterize update_user_policy keys) and #3451 (drop f-string LIMIT in get_simulations) were filed in isolation from a targeted bug audit. Neither was found via a systematic grep of the repo's SQL surface. A repo-wide audit of f-string/.format() SQL construction is overdue to flush out any remaining patterns before the next incident.

What goes wrong

Today's workflow assumes contributors notice string-interpolated SQL during review. There is no:

• Automated check (ruff, bandit, semgrep rule) for execute(f"..." or execute("..." % ...) patterns.
• Pre-commit hook blocking string-interpolated SQL.
• Documented guideline in CONTRIBUTING.md about parameterization.

The result is that new endpoints can quietly reintroduce injection vectors; #3445 and #3451 both survived multiple reviews.

Suggested fix

1. Audit: grep the repo for all SQL execution sites (cursor.execute, db.execute, connection.execute, etc.) and verify every dynamic segment is a ?/:param placeholder. Cover at minimum:
   • policyengine_api/endpoints/ (all files)
   • policyengine_api/services/
   • policyengine_api/data/
   • policyengine_api/utils/
2. Codify: add a bandit config or semgrep rule to CI that fails on execute(f"..." and similar patterns. Example rule:

- id: fstring-sql
  pattern-either:
    - pattern: $CONN.execute(f"...")
    - pattern: $CONN.execute("..." % $X)
    - pattern: $CONN.execute("..." + $X)
  message: Don't build SQL with string interpolation — use parameterized queries.
  severity: ERROR
  languages: [python]

3. Document in CONTRIBUTING.md: "SQL must use parameterized queries; never f"..." or .format() into a SQL string."

Severity

Medium — no known active vulnerability, but the class of bug has now resurfaced twice in the same repo.

Relates to

Fixes #3445, #3451 (both closed).

Start by grepping cursor.execute, db.execute, and connection.execute across policyengine_api/endpoints/, services/, data/, and utils/, then inspect the existing CI configuration. Audit each dynamic SQL construction, add a CI rule covering the listed interpolation patterns, and update CONTRIBUTING.md with the parameterization guideline. Done means the audited sites use placeholders, CI rejects the patterns, and the guideline is documented.