Fixes#72
## Why
`compilePatterns` silently discards any pattern that fails to compile. A typo in a blocklist entry therefore turns a deny rule into a no-op — a fail-open with security impact.
## Changes
- Add `Remote.ValidatePatterns`, which compiles every pattern list (patterns, blocklist, mutable/immutable patterns, ban_tags) and returns an error on the first invalid regex.
- Reject invalid patterns with 400 at remote create and update time.
- Unit test for valid and invalid patterns.
## Validation
- `go test ./pkg/models/` and `make e2e` pass.
Reviewed-on: #87
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>
