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

pkg/api/v0/types.go

@@ -15,23 +15,29 @@ type RegistryExtensions struct {
 	ReleaseDate string    `json:"release_date"`
 }
 
-// ServerRecord represents the unified storage and API response model
-type ServerRecord struct {
-	Server                          model.ServerJSON       `json:"server"`                                                    // Pure MCP server.json
-	XIOModelContextProtocolRegistry RegistryExtensions     `json:"x-io.modelcontextprotocol.registry,omitempty"` // Registry-generated data
-	XPublisher                      map[string]interface{} `json:"x-publisher,omitempty"`                       // x-publisher extensions
-}
-
 // ServerListResponse represents the paginated server list response
 type ServerListResponse struct {
-	Servers  []ServerRecord `json:"servers"`
-	Metadata *Metadata      `json:"metadata,omitempty"`
+	Servers  []ServerJSON `json:"servers"`
+	Metadata *Metadata    `json:"metadata,omitempty"`
 }
 
-// PublishRequest represents the API request format for publishing servers
-type PublishRequest struct {
-	Server     model.ServerJSON       `json:"server"`
-	XPublisher map[string]interface{} `json:"x-publisher,omitempty"`
+// ServerMeta represents the structured metadata with known extension fields
+type ServerMeta struct {
+	Publisher                      map[string]interface{} `json:"publisher,omitempty"`
+	IOModelContextProtocolRegistry *RegistryExtensions    `json:"io.modelcontextprotocol.registry,omitempty"`
+}
+
+// ServerJSON represents complete server information as defined in the MCP spec, with extension support
+type ServerJSON struct {
+	Schema        string              `json:"$schema,omitempty"`
+	Name          string              `json:"name" minLength:"1" maxLength:"200"`
+	Description   string              `json:"description" minLength:"1" maxLength:"100"`
+	Status        model.Status        `json:"status,omitempty" minLength:"1"`
+	Repository    model.Repository    `json:"repository,omitempty"`
+	VersionDetail model.VersionDetail `json:"version_detail"`
+	Packages      []model.Package     `json:"packages,omitempty"`
+	Remotes       []model.Remote      `json:"remotes,omitempty"`
+	Meta          *ServerMeta         `json:"_meta,omitempty"`
 }
 
 // Metadata represents pagination metadata