Fix tag handling for multi-word tags

Tags with spaces (e.g. #science fiction) were breaking extraction because
TAG_RE only matched word characters — it would stop at the space and extract
#science instead of #science-fiction.

- TAG_RE (backend + frontend): add hyphens to character class so #science-fiction
  is recognized as a single tag: [\w][\w-]* per segment
- System prompt: instruct LLM to use hyphens in multi-word tags, never spaces
- tag_suggestions.py: update prompt example + sanitize output by replacing
  spaces with hyphens as a safety net regardless of LLM output
- append-tag route: sanitize incoming tag (spaces → hyphens) before appending

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/utils/tags.py

@@ -1,7 +1,7 @@
 import re
 
 _CODE_FENCE_RE = re.compile(r"```[\s\S]*?```|`[^`\n]+`")
-_TAG_RE = re.compile(r"(?<!\w)#([\w]+(?:/[\w]+)*)")
+_TAG_RE = re.compile(r"(?<!\w)#([\w][\w-]*(?:/[\w][\w-]*)*)")
 
 
 def extract_tags(body: str) -> list[str]: