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/templates/partials/journal_day_detail.html

@@ -14,6 +14,11 @@
         editing: {{ 'true' if editing_entry_id == item.id else 'false' }},
         moods: [{% for m in item.moods %}'{{ m.value }}'{{ ',' if not loop.last }}{% endfor %}],
         toggleMood(v) { this.moods.includes(v) ? this.moods = this.moods.filter(m => m !== v) : this.moods.push(v) },
+        previewMode: false,
+        togglePreview() {
+          this.previewMode = !this.previewMode;
+          if (this.previewMode) { this.$refs.contentField.dispatchEvent(new Event('input')); }
+        },
       }"
     >
       <div x-show="!editing">
@@ -24,7 +29,7 @@
           {% endif %}
         </div>
         {% if item.title %}<div class="journal-entry-title">{{ item.title }}</div>{% endif %}
-        <div class="journal-entry-content">{{ item.content }}</div>
+        <div class="journal-entry-content">{{ item.content|markdown }}</div>
         {% if item.tags %}
         <div class="journal-entry-tags">
           {% for tag in item.tags %}<span class="tag-chip">#{{ tag }}</span>{% endfor %}
@@ -94,7 +99,22 @@
         </div>
         <div class="field">
           <label>내용</label>
-          <textarea name="content" rows="4" required>{{ item.content }}</textarea>
+          {% include "partials/journal_editor_toolbar.html" %}
+          <div x-show="!previewMode">
+            <textarea
+              name="content"
+              rows="6"
+              required
+              class="markdown-editor"
+              x-ref="contentField"
+              hx-post="/journal/preview"
+              hx-trigger="input changed delay:400ms, load"
+              hx-target="#journal-edit-preview-{{ item.id }}"
+              hx-swap="innerHTML"
+              hx-params="content"
+            >{{ item.content }}</textarea>
+          </div>
+          <div id="journal-edit-preview-{{ item.id }}" class="journal-preview journal-entry-content" x-show="previewMode" x-cloak></div>
         </div>
         <div class="field">
           <label>기분 (여러 개 선택 가능)</label>