v0.3.1 — default-install fix (DE-305)

Patch release over v0.3.0.

Fix

• A default docker compose up no longer fails on a fresh install. The opt-in Slack/Teams intake-bridge env vars were declared with the required-error ${VAR:?} form, which Docker Compose evaluates before --profile filtering — so a plain docker compose up (no bridge profile) aborted with required variable SLACK_CLIENT_ID is missing a value, contradicting .env.example. The 8 bridge vars now use ${VAR:-}; the "required when the profile is active" guarantee moved into each bridge's Settings (a Pydantic field validator that fails fast at bridge startup, only when the profile is enabled). Community-reported as #92; tracked as DE-305. (#96)
• Release workflow SBOM job authenticates to GHCR (#95).

Notes

• api + gateway both report 0.3.1.
• No application-code change beyond the version string; the fix is in docker-compose.yml + the bridge services.

Thanks to @legalinnovationaggregator-coder for the precise bug report.