Fix validated security findings

[sgrastar]

Summary

• remediate all six findings from the September 3, 2026 Codex Security report
• bound hosted transcript ingestion, reconciliation, downloads, and metadata generation by persistent quotas, fair work admission, and atomic owner/run/global rate limits
• preserve authenticated preloaded-generation ordering, fail closed on legacy environment conflicts, evaluate every applicable logout request, scope localized UI-language uniqueness correctly, and generate document-wide unique metadata IDs
• pin the canonical Caddy-to-container proxy boundary to an explicit trusted Docker gateway so client identities cannot collapse or be spoofed
• renew the signed G2 approval record against the exact remediation commit after an independent attack-path review approved all six fixes

Verification

• ./gradlew check
• .venv/bin/python tools/g1_docgen.py --check
• .venv/bin/python tools/g1_validate.py --structural-only (46/46 PASS)
• .venv/bin/python tools/g2_validate.py (21/21 PASS)
• G2_TOOLS_COMMIT=5aa7c82637472c01d538f4345d5bccd9c67ac2bb PY=.venv/bin/python tools/g2_ci_verify.sh (21/21 PASS, external-pin)
• docker compose -f deploy/compose.yaml config --quiet
• SSH signatures verified for both commits