test: add self-contained integration environment

Separate schema migrations from environment-specific fixtures, run the full API workflow against a disposable PostgreSQL Testcontainer, and provide seeded development and UI test environments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/migrations/00005_question_code_snippet.sql

@@ -2,9 +2,5 @@
 ALTER TABLE questions
     ADD COLUMN code_snippet TEXT NOT NULL DEFAULT '';
 
-UPDATE questions
-SET code_snippet = E'function CandidateList({ candidates }) {\n  return candidates.map(candidate => (\n    <CandidateCard candidate={candidate} />\n  ));\n}'
-WHERE type = 'code_review' AND code_snippet = '';
-
 -- +goose Down
 ALTER TABLE questions DROP COLUMN code_snippet;