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>

frontend/src/utils/tags.ts

@@ -1,5 +1,5 @@
 const CODE_FENCE_RE = /```[\s\S]*?```|`[^`\n]+`/g;
-const TAG_RE = /(?<!\w)(?<!&)#([\w]+(?:\/[\w]+)*)/g;
+const TAG_RE = /(?<!\w)(?<!&)#([\w][\w-]*(?:\/[\w][\w-]*)*)/g;
 
 function escapeHtmlAttr(s: string): string {
   return s.replace(/&/g, "&amp;").replace(/"/g, "&quot;").replace(/</g, "&lt;").replace(/>/g, "&gt;");