fix(security): replace naive SQL validator with sqlglot AST check

[gordonmurray]

Summary

The previous validator in /api/query uppercased the whole query and blocked it if any of DELETE / DROP / INSERT / UPDATE / CREATE / ALTER appeared as a substring. That:

• over-blocked legitimate queries — SELECT * FROM update_log, WHERE status = 'create'
• was bypassable — comments, unusual whitespace, or string literals could slip through

This PR replaces it with a sqlglot-based AST validator.

How it works

• Parses the SQL with sqlglot.parse(..., dialect="duckdb") and rejects parse errors, empty input, and multi-statement inputs
• Accepts only SELECT / WITH / UNION / INTERSECT / EXCEPT / VALUES at the top level
• Walks the entire tree and rejects any Insert / Update / Delete / Create / Drop / Alter / TruncateTable / Commit / Rollback / Transaction / Use / Attach / Detach / Merge / Copy / Command node, even nested in subqueries or CTEs
• Injects an outer LIMIT via AST manipulation if the user didn't supply one — replaces the old _limit_query string-append

Also cleans up the /api/query error flow so 400s raised by the validator are re-raised unchanged rather than re-wrapped as "Query execution failed: 400: ...".

Adds sqlglot==30.4.3 to backend/requirements.txt.

Verification (done locally against the demo stack)

Scenario	Before	After
Happy path query	37,537 rows	37,537 rows
WITH update_log AS (...) SELECT * FROM update_log	rejected (false positive)	accepted
SELECT 1 WHERE 'create' = 'create'	rejected (false positive)	accepted
DROP TABLE x	400	400
INSERT INTO x VALUES (1)	400	400
UPDATE x SET a=1	400	400
DELETE FROM x	400	400
CREATE TABLE y AS SELECT 1	400	400
ALTER TABLE x ...	400	400
ATTACH 'evil.db'	accepted (old bypass)	400
COPY x TO 's3://evil'	accepted (old bypass)	400
SELECT 1; DROP TABLE x	rejected (substring match)	400 (multi-statement)
/*DROP*/ SELECT 1	rejected (substring match)	accepted (comment stripped)
SELECT FROM (malformed)	accepted → DuckDB error	400 (parse error)
Error message format	Query execution failed: 400: Only SELECT...	Only SELECT queries are allowed (got DROP)
rowLimit=3, no explicit LIMIT	works	works (3 rows, truncated=true)
User LIMIT 2, rowLimit=10	works	works (2 rows, user LIMIT preserved)

Test plan for reviewer

• docker compose down -v && docker compose up --build
• Click through the five Sample Queries — all succeed
• Paste DROP TABLE x and run — expect 400 Only SELECT queries are allowed (got DROP)
• Paste SELECT * FROM update_log (with a real CTE) — expect success
• CI goes green (lint + two docker builds)

Closes #6