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/model/types.go

@@ -9,17 +9,6 @@ const (
 	StatusDeleted    Status = "deleted"
 )
 
-// ServerJSON represents complete server information as defined in the MCP spec (pure, no registry metadata)
-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        Status        `json:"status,omitempty" minLength:"1"`
-	Repository    Repository    `json:"repository,omitempty"`
-	VersionDetail VersionDetail `json:"version_detail"`
-	Packages      []Package     `json:"packages,omitempty"`
-	Remotes       []Remote      `json:"remotes,omitempty"`
-}
 
 // Package represents a package configuration
 type Package struct {