fix: blocklist fails open when a regex fails to compile (#87)

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>

internal/api/v2/remotes.go

@@ -69,6 +69,10 @@ func (h *RemotesHandler) create(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "base_url is required for remote repositories", http.StatusBadRequest)
 		return
 	}
+	if err := remote.ValidatePatterns(); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
 	if err := h.db.CreateRemote(r.Context(), &remote); err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
@@ -84,6 +88,10 @@ func (h *RemotesHandler) update(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	remote.Name = name
+	if err := remote.ValidatePatterns(); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
 	if err := h.db.UpdateRemote(r.Context(), &remote); err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return