Harden auth and secret handling

[likawa3b]

Summary

• restrict legacy env-admin auth fallback to explicit single-user mode
• mask password/secret settings responses and expand agent audit redaction keys
• reject inline strategy exchange secrets unless using credential_id, and redact persisted strategy config responses
• bind frontend compose port to localhost by default and ignore local secret/review artifacts

Validation

• docker pytest: 40 passed, 3 warnings
• docker compileall for changed backend modules: passed
• git diff --check: passed
• local deploy: backend/frontend rebuilt and healthy at http://127.0.0.1:8889
• deployed smoke: frontend 200, admin login code=1, settings values returned no raw secret fields

Reviewer Pass

• ponytail-review: scoped pass, no over-engineering issue requiring patch
• claude/gemini/glm wrapper attempted; Gemini failed inside grep_search regex, Claude returned max-turns only, GLM failed before review due WSL timeout; no actionable reviewer findings were produced