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

tools/validate-examples/main.go

@@ -14,9 +14,9 @@ import (
 	"regexp"
 	"strings"
 
-	jsonschema "github.com/santhosh-tekuri/jsonschema/v5"
-	"github.com/modelcontextprotocol/registry/pkg/model"
 	"github.com/modelcontextprotocol/registry/internal/validators"
+	apiv0 "github.com/modelcontextprotocol/registry/pkg/api/v0"
+	jsonschema "github.com/santhosh-tekuri/jsonschema/v5"
 )
 
 const (
@@ -129,26 +129,26 @@ func validateAgainstSchema(data any, schema *jsonschema.Schema, schemaName strin
 }
 
 func validateWithObjectValidator(serverData any) bool {
-	var serverDetail model.ServerJSON
+	var serverDetail apiv0.ServerJSON
 	serverDataBytes, err := json.Marshal(serverData)
 	if err != nil {
 		log.Printf("  Validating with Go Validator: ❌")
 		log.Printf("    Error marshaling server data: %v", err)
 		return false
 	}
-	
+
 	if err := json.Unmarshal(serverDataBytes, &serverDetail); err != nil {
 		log.Printf("  Validating with Go Validator: ❌")
 		log.Printf("    Error unmarshaling to ServerDetail: %v", err)
 		return false
 	}
-	
+
 	if err := validators.ValidateServerJSON(&serverDetail); err != nil {
 		log.Printf("  Validating with Go Validator: ❌")
 		log.Printf("    Error: %v", err)
 		return false
 	}
-	
+
 	log.Printf("  Validating with Go Validator: ✅")
 	return true
 }