feat: add SymposiumSeeder and MessengerSeeder bridge interfaces

Defines cross-plugin seeder interfaces in the SDK so template plugins
can seed Symposium/Messenger content via PluginBridge without importing
their database packages directly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugin/seeder.go

@@ -0,0 +1,105 @@
+package plugin
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5"
+)
+
+// SymposiumSeeder provides Symposium data-seeding and lookup operations for
+// cross-plugin use via PluginBridge. Template plugins retrieve this interface
+// instead of importing the Symposium database package directly.
+//
+// All Seed* methods are idempotent: existing rows are returned, not duplicated.
+type SymposiumSeeder interface {
+	// Wiki
+	SeedWikiCategory(ctx context.Context, tx pgx.Tx, name, slug, description string, sortOrder int32) (string, error)
+	SeedWikiArticle(ctx context.Context, tx pgx.Tx, title, slug, categoryID, excerpt string, contentJSON []byte) error
+
+	// Courses
+	SeedCourse(ctx context.Context, tx pgx.Tx, p SeedCourseParams) (string, error)
+	SeedCourseSection(ctx context.Context, tx pgx.Tx, courseID, title string, position int32) (string, error)
+	SeedLesson(ctx context.Context, tx pgx.Tx, courseID, sectionID string, p SeedLessonParams) (string, error)
+
+	// Quizzes
+	SeedQuiz(ctx context.Context, tx pgx.Tx, title string, passThreshold int32) (string, error)
+	SeedQuizQuestion(ctx context.Context, tx pgx.Tx, quizID string, p SeedQuizQuestionParams) (string, error)
+
+	// Community
+	SeedCommunityCategory(ctx context.Context, tx pgx.Tx, name, slug, description, icon string, position, minTierLevel int32) (string, error)
+
+	// Course categories & tags
+	SeedCourseCategory(ctx context.Context, tx pgx.Tx, name, slug string, description *string, position int32) error
+	SeedCourseTag(ctx context.Context, tx pgx.Tx, name, slug string, position *int32) error
+
+	// Lookups — return the entity ID or an error if not found.
+	GetCourseBySlug(ctx context.Context, tx pgx.Tx, slug string) (string, error)
+	GetCommunityCategoryBySlug(ctx context.Context, tx pgx.Tx, slug string) (string, error)
+
+	// Runtime queries
+	GetUserCourseProgress(ctx context.Context, pool Pool, userID string) ([]CourseProgressItem, error)
+}
+
+// SeedCourseParams holds the parameters for seeding a course.
+type SeedCourseParams struct {
+	Title                    string
+	Slug                     string
+	Description              string
+	CertificateEnabled       bool
+	MinTierLevel             int32
+	DifficultyLevel          string
+	EstimatedDurationMinutes int32
+}
+
+// SeedLessonParams holds the parameters for seeding a lesson.
+type SeedLessonParams struct {
+	Title          string
+	Slug           string
+	LessonType     string
+	ContentJSON    []byte
+	QuizID         string
+	Position       int32
+	IsFreePreview  bool
+	CompletionMode string
+}
+
+// SeedQuizQuestionParams holds the parameters for seeding a quiz question.
+type SeedQuizQuestionParams struct {
+	QuestionType  string
+	QuestionText  string
+	Options       []map[string]any
+	CorrectAnswer string
+	Explanation   string
+	Position      int32
+	Rubric        string
+	ScenarioText  string
+}
+
+// CourseProgressItem is a lightweight view of a user's progress in one course.
+type CourseProgressItem struct {
+	CourseTitle   string
+	CourseSlug    string
+	NextLesson   string
+	DoneCount    int
+	TotalLessons int
+}
+
+// MessengerSeeder provides Messenger data-seeding operations for cross-plugin
+// use via PluginBridge. Template plugins retrieve this interface instead of
+// importing the Messenger database package directly.
+type MessengerSeeder interface {
+	CountMessagesForPair(ctx context.Context, tx pgx.Tx, participantPairID string) (int64, error)
+	InsertMessage(ctx context.Context, tx pgx.Tx, p InsertMessageParams) error
+}
+
+// InsertMessageParams holds the parameters for inserting a messenger message.
+type InsertMessageParams struct {
+	SenderType           string
+	SenderID             string
+	SenderDisplayName    string
+	RecipientType        string
+	RecipientID          string
+	RecipientDisplayName string
+	BodyMarkdown         string
+	ParticipantPairID    string
+}