Simplify templates to pure Jinja2 + CodeMirror editor + variable reference

Major template system overhaul:
- TemplateConfig simplified from 21 fields to 9: removed all sub-templates
  (asset_image, asset_video, assets_format, people_format, etc.)
  Users write full Jinja2 with {% for %}, {% if %} inline.
- Default EN/RU templates seeded on first startup (user_id=0, system-owned)
  with proper Jinja2 loops over added_assets, people, albums.
- build_full_context() simplified: passes raw data directly to Jinja2
  instead of pre-rendering sub-templates.
- CodeMirror editor for template slots (HTML syntax highlighting,
  line wrapping, dark theme support via oneDark).
- Variable reference API: GET /api/template-configs/variables returns
  per-slot variable descriptions + asset_fields for loop contexts.
- Variable reference modal in UI: click "{{ }} Variables" next to any
  slot to see available variables with Jinja2 syntax examples.
- Route ordering fix: /variables registered before /{config_id}.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/JinjaEditor.svelte

@@ -0,0 +1,63 @@
+<script lang="ts">
+	import { onMount } from 'svelte';
+	import { EditorView, keymap, placeholder as cmPlaceholder } from '@codemirror/view';
+	import { EditorState } from '@codemirror/state';
+	import { html } from '@codemirror/lang-html';
+	import { oneDark } from '@codemirror/theme-one-dark';
+	import { getTheme } from '$lib/theme.svelte';
+
+	let { value = '', onchange, rows = 6, placeholder = '' } = $props<{
+		value: string;
+		onchange: (val: string) => void;
+		rows?: number;
+		placeholder?: string;
+	}>();
+
+	let container: HTMLDivElement;
+	let view: EditorView;
+	const theme = getTheme();
+
+	onMount(() => {
+		const extensions = [
+			html(),  // Jinja2 is close enough to HTML template syntax for highlighting
+			EditorView.updateListener.of((update) => {
+				if (update.docChanged) {
+					onchange(update.state.doc.toString());
+				}
+			}),
+			EditorView.lineWrapping,
+			EditorView.theme({
+				'&': { fontSize: '13px', fontFamily: 'monospace' },
+				'.cm-content': { minHeight: `${rows * 1.5}em`, padding: '8px' },
+				'.cm-editor': { borderRadius: '0.375rem', border: '1px solid var(--color-border)' },
+				'.cm-focused': { outline: '2px solid var(--color-primary)', outlineOffset: '0px' },
+			}),
+		];
+
+		if (theme.isDark) {
+			extensions.push(oneDark);
+		}
+
+		if (placeholder) {
+			extensions.push(cmPlaceholder(placeholder));
+		}
+
+		view = new EditorView({
+			state: EditorState.create({ doc: value, extensions }),
+			parent: container,
+		});
+
+		return () => view.destroy();
+	});
+
+	// Sync external value changes (e.g. when editing different config)
+	$effect(() => {
+		if (view && view.state.doc.toString() !== value) {
+			view.dispatch({
+				changes: { from: 0, to: view.state.doc.length, insert: value },
+			});
+		}
+	});
+</script>
+
+<div bind:this={container}></div>