Implement flattened _meta format for server extensions (#331)

## Summary

This PR implements the flattened `_meta` format for server extensions as
discussed in #284, replacing the confusing wrapper format with a clean
single-file approach.

**Before (wrapper format):**
```json
{
  "server": { "name": "my-server", "description": "..." },
  "x-publisher": { "build_info": "..." },
  "x-io.modelcontextprotocol.registry": { "id": "...", "published_at": "..." }
}
```

**After (flattened format):**
```json
{
  "name": "my-server",
  "description": "...",
  "_meta": {
    "publisher": { "build_info": "..." },
    "io.modelcontextprotocol.registry": { "id": "...", "published_at": "..." }
  }
}
```

## Key Changes

- **Flattened Format**: Single `ServerJSON` file with structured `_meta`
field
- **Better DX**: Publishers maintain one file instead of separate
server.json + extensions.json
- **MCP Compliance**: Uses standard `_meta` mechanism instead of `x-*`
prefixes
- **Simplified Database**: PostgreSQL now uses simple `id, value` table
structure
- **Type Safety**: Structured `ServerMeta` with known extension fields

## Database Migration

Added migration `003_simplify_to_key_value.sql` that drops the complex
relational structure and creates a simple key-value table. This
significantly simplifies database management.

## Testing

- All core tests passing (validators, service layer)
- Schema validation working for all 10 examples
- Seed data successfully migrated (396 entries)
- Publishing functionality verified

Fixes #284

cmd/publisher/commands/init.go

@@ -11,10 +11,10 @@ import (
 	"strings"
 	"time"
 
+	apiv0 "github.com/modelcontextprotocol/registry/pkg/api/v0"
 	"github.com/modelcontextprotocol/registry/pkg/model"
 )
 
-
 func InitCommand() error {
 	// Check if server.json already exists
 	if _, err := os.Stat("server.json"); err == nil {
@@ -266,7 +266,7 @@ func createServerJSON(
 	name, description, version, repoURL, repoSource,
 	packageType, packageIdentifier, packageVersion string,
 	envVars []model.KeyValueInput,
-) model.ServerJSON {
+) apiv0.ServerJSON {
 	// Determine registry type and base URL
 	var registryType, registryBaseURL string
 	switch packageType {
@@ -297,7 +297,7 @@ func createServerJSON(
 	}
 
 	// Create server structure
-	return model.ServerJSON{
+	return apiv0.ServerJSON{
 		Schema:      "https://static.modelcontextprotocol.io/schemas/2025-07-09/server.schema.json",
 		Name:        name,
 		Description: description,

cmd/publisher/commands/publish.go

@@ -11,6 +11,8 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+
+	apiv0 "github.com/modelcontextprotocol/registry/pkg/api/v0"
 )
 
 func PublishCommand(args []string) error {
@@ -30,7 +32,7 @@ func PublishCommand(args []string) error {
 	}
 
 	// Validate JSON
-	var serverJSON map[string]any
+	var serverJSON apiv0.ServerJSON
 	if err := json.Unmarshal(serverData, &serverJSON); err != nil {
 		return fmt.Errorf("invalid server.json: %w", err)
 	}
@@ -73,26 +75,14 @@ func PublishCommand(args []string) error {
 
 func publishToRegistry(registryURL string, serverData []byte, token string) error {
 	// Parse the server JSON data
-	var serverDetails map[string]any
-	err := json.Unmarshal(serverData, &serverDetails)
+	var serverJSON apiv0.ServerJSON
+	err := json.Unmarshal(serverData, &serverJSON)
 	if err != nil {
 		return fmt.Errorf("error parsing server.json file: %w", err)
 	}
 
-	// Create the publish request payload
-	var publishReq map[string]any
-	if _, hasServerField := serverDetails["server"]; hasServerField {
-		// Already in PublishRequest format
-		publishReq = serverDetails
-	} else {
-		// Wrap in server field
-		publishReq = map[string]any{
-			"server": serverDetails,
-		}
-	}
-
 	// Convert to JSON
-	jsonData, err := json.Marshal(publishReq)
+	jsonData, err := json.Marshal(serverJSON)
 	if err != nil {
 		return fmt.Errorf("error serializing request: %w", err)
 	}
@@ -129,4 +119,4 @@ func publishToRegistry(registryURL string, serverData []byte, token string) erro
 	}
 
 	return nil
-}
+}

cmd/registry/main.go

@@ -16,7 +16,7 @@ import (
 	"github.com/modelcontextprotocol/registry/internal/database"
 	"github.com/modelcontextprotocol/registry/internal/service"
 	"github.com/modelcontextprotocol/registry/internal/telemetry"
-	"github.com/modelcontextprotocol/registry/pkg/model"
+	apiv0 "github.com/modelcontextprotocol/registry/pkg/api/v0"
 )
 
 func main() {
@@ -46,7 +46,7 @@ func main() {
 	// Initialize services based on environment
 	switch cfg.DatabaseType {
 	case config.DatabaseTypeMemory:
-		db = database.NewMemoryDB(map[string]*model.ServerJSON{})
+		db = database.NewMemoryDB(map[string]*apiv0.ServerJSON{})
 		registryService = service.NewRegistryServiceWithDB(db)
 	case config.DatabaseTypePostgreSQL:
 		// Use PostgreSQL for real registry service