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/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