Split Purger into single-row and batch patterns

## Overview

Refactor Purger pattern to separate single-row and batch deletion operations for consistency with Updater/BatchUpdater pattern.

## Changes

### Current State

- `Purger`: Batch delete with subquery and batch_size
- `PurgeTarget`: Defines pk_column and build_subquery
- `execute_purger`: Executes batched deletion

### Target State

**Single-row deletion (Purger)**:

```python
class PurgerSpec(ABC, Generic[TRow]):
@property
def row_class(self) -> type[TRow]: ...

@dataclass
class Purger(Generic[TRow]):
spec: PurgerSpec[TRow]
pk_value: UUID | str | int

@dataclass
class PurgerResult(Generic[TRow]):
row: TRow # Deleted row via RETURNING

async def execute_purger(...) -> PurgerResult[TRow] | None
```

**Batch deletion (BatchPurger)**:

```python
class BatchPurgerSpec(ABC): # Current PurgeTarget
@property
def pk_column(self) -> sa.Column: ...
def build_subquery(self) -> sa.sql.Select: ...

@dataclass
class BatchPurger: # Current Purger
spec: BatchPurgerSpec
batch_size: int = 1000

@dataclass
class BatchPurgerResult:
deleted_count: int

async def execute_batch_purger(...) -> BatchPurgerResult
```

## Tasks

- [ ] Create PurgerSpec and Purger for single-row delete by PK
- [ ] Create PurgerResult that returns deleted row
- [ ] Implement execute_purger for single-row deletion
- [ ] Rename current PurgeTarget to BatchPurgerSpec
- [ ] Rename current Purger to BatchPurger
- [ ] Rename current PurgerResult to BatchPurgerResult
- [ ] Rename execute_purger to execute_batch_purger
- [ ] Update **init**.py exports
- [ ] Add/update tests

JIRA Issue: BA-3350