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/journal.html

@@ -124,11 +124,20 @@
     categoryTemplates: {{ category_templates|tojson|forceescape }},
     content: '',
     lastAutoFilled: '',
+    previewMode: false,
+    togglePreview() {
+      this.previewMode = !this.previewMode;
+      if (this.previewMode) { this.$refs.contentField.dispatchEvent(new Event('input')); }
+    },
     onCategoryChange(categoryId) {
       const tmpl = this.categoryTemplates[categoryId] || '';
       if (this.content === this.lastAutoFilled) {
         this.content = tmpl;
         this.lastAutoFilled = tmpl;
+        this.$nextTick(() => {
+          if (this.$refs.contentField._easymde) { this.$refs.contentField._easymde.value(tmpl); }
+          this.$refs.contentField.dispatchEvent(new Event('input'));
+        });
       }
     },
     init() { this.onCategoryChange(this.$refs.categorySelect.value); },
@@ -173,7 +182,25 @@
 
     <div class="field">
       <label for="new-entry-content">내용</label>
-      <textarea id="new-entry-content" name="content" rows="8" required placeholder="오늘 하루는 어땠나요?" x-model="content"></textarea>
+      {% include "partials/journal_editor_toolbar.html" %}
+      <div x-show="!previewMode">
+        <textarea
+          id="new-entry-content"
+          name="content"
+          rows="8"
+          required
+          placeholder="오늘 하루는 어땠나요?"
+          class="markdown-editor"
+          x-model="content"
+          x-ref="contentField"
+          hx-post="/journal/preview"
+          hx-trigger="input changed delay:400ms, load"
+          hx-target="#new-entry-preview"
+          hx-swap="innerHTML"
+          hx-params="content"
+        ></textarea>
+      </div>
+      <div id="new-entry-preview" class="journal-preview journal-entry-content" x-show="previewMode" x-cloak></div>
     </div>
 
     <div class="field">