Fix lyrics Unicode corruption and escaped quotes in Genius plugin

## Problem
The lyrics plugin has two bugs that corrupt fetched lyrics:

1. **Unicode corruption**: Characters like `ò`, `è`, `à` are corrupted to `√≤`, `√®`, etc.
2. **Escaped quotes**: Quotes appear as `\"` instead of `"` in lyrics

## Root Causes

### Issue 1: MacRoman encoding misdetection
- **Location**: `RequestHandler.fetch_text()` line 220
- **Cause**: Setting `r.encoding = None` forces requests to use `apparent_encoding`
- **Problem**: For Genius.com (and others), requests incorrectly detects MacRoman instead of UTF-8
- **Result**: UTF-8 bytes `c3 b2` (ò) decoded as MacRoman produces "√≤" (U+221A U+2264)

### Issue 2: Incomplete JSON unescape
- **Location**: `Genius.scrape()` line 576
- **Cause**: The `remove_backslash` regex doesn't handle all escape patterns in JSON
- **Problem**: Genius embeds lyrics in JSON with patterns like `\\"` and `\\\\"` 
- **Result**: After BeautifulSoup processing, escaped quotes remain in final text

## Solution

### Fix 1: Trust server encoding, fallback to UTF-8
```python
# OLD: r.encoding = None
# NEW:
if not r.encoding:
    r.encoding = 'utf-8'
```
- Respects server's declared encoding (UTF-8 for Genius)
- Falls back to UTF-8 if no encoding specified (safer than apparent_encoding)
- Preserves original intent of handling misconfigured servers

### Fix 2: Iteratively clean escaped quotes
```python
while '\\"' in lyrics:
    lyrics = lyrics.replace('\\"', '"')
```
- Handles variable escape levels (`\"`, `\\\"`, `\\\\\"`)
- Minimal change - keeps original `remove_backslash` regex
- Applied after BeautifulSoup to avoid interfering with HTML parsing

## Testing

Tested with:
- Caparezza - "Argenti Vive" (Italian, many accented characters)
- WestsideGunn - "Heel Cena" (escaped quotes in lyrics)

Before:
```
mi si parò davanti
\\"I got big moves\\"
```

After:
```
mi si parò davanti
"I got big moves"
```

## Impact
- Fixes lyrics for all languages with non-ASCII characters
- Fixes Genius lyrics with quotes
- No breaking changes - maintains backward compatibility
- Minimal code changes (14 lines total)

beetsplug/lyrics.py

@@ -200,7 +200,11 @@ class LyricsRequestHandler(RequestHandler):
         url = self.format_url(url, params)
         self.debug("Fetching HTML from {}", url)
         r = self.get(url, **kwargs)
-        r.encoding = None
+        """Trust server's encoding,
+        but default to UTF-8 if not specified
+        """
+        if not r.encoding:
+            r.encoding = 'utf-8'
         return r.text
 
     def get_json(self, url: str, params: JSONDict | None = None, **kwargs):
@@ -557,11 +561,14 @@ class Genius(SearchBackend):
     def scrape(cls, html: str) -> str | None:
         if m := cls.LYRICS_IN_JSON_RE.search(html):
             html_text = cls.remove_backslash(m[0]).replace(r"\n", "\n")
-            return cls.get_soup(html_text).get_text().strip()
+            lyrics = cls.get_soup(html_text).get_text().strip()
+            # Clean up any remaining escaped quotes (may need multiple passes)
+            while '\\"' in lyrics:
+                lyrics = lyrics.replace('\\"', '"')
+            return lyrics
 
         return None
 
-
 class Tekstowo(SearchBackend):
     """Fetch lyrics from Tekstowo.pl."""