feat(docker-watcher): phase 5 - registry client & poller

Gitea registry client with tag listing and pattern matching, cron-based
polling scheduler with first-poll safety, poll state persistence.
DeployTriggerer interface for decoupled deploy triggering.

internal/registry/registry.go

@@ -0,0 +1,78 @@
+package registry
+
+import (
+	"context"
+	"fmt"
+	"path"
+	"sort"
+	"strings"
+)
+
+// Client defines the interface for interacting with a container image registry.
+type Client interface {
+	// ListTags returns all available tags for the given image.
+	ListTags(ctx context.Context, image string) ([]string, error)
+
+	// GetLatestTag returns the most recently created tag that matches the given
+	// glob pattern. Returns an empty string and no error if no tags match.
+	GetLatestTag(ctx context.Context, image string, pattern string) (string, error)
+}
+
+// DeployTriggerer is called by the poller when a new tag is detected for a
+// stage with auto_deploy enabled. This decouples the registry package from the
+// deployer implementation.
+type DeployTriggerer interface {
+	TriggerDeploy(ctx context.Context, projectID, stageID, imageTag string) error
+}
+
+// MatchTags filters a list of tags, returning only those that match the given
+// glob pattern. Pattern matching uses path.Match semantics (*, ?, []).
+// Returns an error if the pattern is malformed.
+func MatchTags(tags []string, pattern string) ([]string, error) {
+	if pattern == "" || pattern == "*" {
+		result := make([]string, len(tags))
+		copy(result, tags)
+		return result, nil
+	}
+
+	// Validate pattern once before iterating.
+	if _, err := path.Match(pattern, ""); err != nil {
+		return nil, fmt.Errorf("invalid tag pattern %q: %w", pattern, err)
+	}
+
+	var matched []string
+	for _, tag := range tags {
+		ok, _ := path.Match(pattern, tag)
+		if ok {
+			matched = append(matched, tag)
+		}
+	}
+	return matched, nil
+}
+
+// LatestTag returns the last element of a sorted tag list that matches the
+// pattern. Tags are sorted lexicographically; the "latest" is the last in sort
+// order. Returns empty string if no tags match. Returns an error if the pattern
+// is malformed.
+func LatestTag(tags []string, pattern string) (string, error) {
+	matched, err := MatchTags(tags, pattern)
+	if err != nil {
+		return "", err
+	}
+	if len(matched) == 0 {
+		return "", nil
+	}
+	sort.Strings(matched)
+	return matched[len(matched)-1], nil
+}
+
+// NewClient creates a registry Client based on the registry type string.
+// Supported types: "gitea". Future: "github", "dockerhub".
+func NewClient(registryType, baseURL, token string) (Client, error) {
+	switch strings.ToLower(registryType) {
+	case "gitea":
+		return NewGiteaClient(baseURL, token), nil
+	default:
+		return nil, fmt.Errorf("unsupported registry type: %s", registryType)
+	}
+}