feat: add per-widget secret storage for release provider authentication
Adds encrypted token storage per release widget so users can authenticate
with GitHub, GitLab, Docker Hub, etc. without needing full integration setup.
- New `widget_secret` table (all 3 DB drivers) keyed by (itemId, kind)
- tRPC router with setSecret/deleteSecret/getConfiguredKinds (board modify access required)
- Releases router injects decrypted tokens into provider requests (board view access required)
- Tokens only used for repos matching widget's stored configuration (prevents proxy abuse)
- Provider auth: GitHub (Octokit auth), GitLab (PRIVATE-TOKEN), Docker Hub (Bearer),
GHCR (registry token), npm (Bearer), Codeberg (token header)
- UI: "Provider Tokens" section in widget editor with PasswordInput per provider