journal: real markdown editor (EasyMDE) with live preview toggle, fix default template

- Replace the plain textarea with EasyMDE (vendored locally, no CDN) for
  markdown authoring: syntax highlighting, smart list continuation, and a
  custom text-based toolbar (built-in EasyMDE toolbar icons require Font
  Awesome from a CDN, which this app doesn't use). unorderedListStyle is set
  to "-" to match the app's own template convention.
- Add a preview/edit toggle button that swaps the editor for the exact same
  server-rendered markdown (via /journal/preview) shown after saving, instead
  of always showing both.
- Fix create/edit entry routes to verify the submitted category_id actually
  belongs to the current user before inserting -- every other write path in
  this app already checked ownership; this one didn't (found while manually
  testing the new editor with a typo'd category id that happened to belong to
  someone else's category, which surfaced as an IntegrityError 500 instead of
  a clean 404-equivalent).
- Fix the default "일상" category template: bare "-" bullet lines don't parse
  as list items in the markdown renderer (they need a trailing space), and
  the content_template validator was silently stripping that trailing space
  off on every save. Backfill migration updates any category still holding
  the old, broken template text.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/schemas/journal.py

@@ -29,10 +29,13 @@ class JournalCategoryBase(BaseModel):
     @field_validator("content_template")
     @classmethod
     def blank_template_to_none(cls, v: str | None) -> str | None:
-        if v is None:
+        # color/name과 달리 여기선 .strip()으로 값 자체를 바꾸지 않는다 — 템플릿 맨 끝의
+        # "- "(대시+공백)처럼 의미 있는 trailing whitespace가 있을 수 있고, 그걸 지우면
+        # markdown이 그 줄을 목록으로 인식하지 못하게 된다(빈 값인지 판단만 strip으로 하고,
+        # 실제로 저장하는 값은 원본을 그대로 쓴다).
+        if v is None or not v.strip():
             return None
-        v = v.strip()
-        return v or None
+        return v
 
 
 class JournalCategoryCreate(JournalCategoryBase):