Implement schema registry with auto-detection

[MALathon]

Summary

Create a registry system to store, retrieve, and auto-detect schemas based on URLs.

Design

# fetcharoo/schemas/registry.py
from typing import Dict, Optional, List
from .base import SiteSchema

_SCHEMAS: Dict[str, SiteSchema] = {}

def register_schema(schema: SiteSchema) -> None:
    """Register a schema in the global registry."""
    if schema.name in _SCHEMAS:
        raise ValueError(f"Schema '{schema.name}' already registered")
    _SCHEMAS[schema.name] = schema

def get_schema(name: str) -> Optional[SiteSchema]:
    """Get schema by name."""
    return _SCHEMAS.get(name)

def detect_schema(url: str) -> Optional[SiteSchema]:
    """Auto-detect schema from URL by testing all registered patterns."""
    for schema in _SCHEMAS.values():
        if schema.matches(url):
            return schema
    return None

def list_schemas() -> List[str]:
    """List all registered schema names."""
    return list(_SCHEMAS.keys())

def get_all_schemas() -> Dict[str, SiteSchema]:
    """Get all registered schemas."""
    return _SCHEMAS.copy()

def clear_registry() -> None:
    """Clear all schemas (mainly for testing)."""
    _SCHEMAS.clear()

# Decorator for easy class-based registration
def schema(cls):
    """Decorator to register a schema class."""
    instance = cls() if isinstance(cls, type) else cls
    register_schema(instance)
    return cls

Usage

from fetcharoo.schemas import register_schema, detect_schema, SiteSchema

# Register directly
my_schema = SiteSchema(name="mysite", url_pattern=r"https://mysite\.com/.*")
register_schema(my_schema)

# Or use decorator
@schema
class MySiteSchema(SiteSchema):
    name = "mysite"
    url_pattern = r"https://mysite\.com/.*"

# Auto-detect
schema = detect_schema("https://mysite.com/docs")

Tasks

• Implement registry module with _SCHEMAS dict
• Add register_schema(), get_schema(), list_schemas()
• Implement detect_schema() with URL matching
• Add @schema decorator for class-based registration
• Add clear_registry() for test isolation
• Export from fetcharoo.schemas
• Add unit tests for all registry functions

Acceptance Criteria

• Can register schemas by instance or decorator
• detect_schema() returns correct schema for matching URLs
• Returns None for unrecognized URLs
• list_schemas() shows all registered names

Dependencies

• Create SiteSchema base dataclass #11 (SiteSchema base class)

Part of

Parent issue: #10