重构回忆录为 story-first / markdown-first 架构并整合图片意图与前端 UI 修复

本次 squash merge 将 codex-story-first-image-intent 的整体改动合入 development，核心内容包括：

1. 后端数据与迁移：新增 stories、story_versions、story_image_intents、chapter_cover_intents、assets 等模型与 Alembic 迁移，建立 story-first、markdown-first、asset-first 的主数据链路。

2. 生成与任务链：引入 StoryBuilderOrchestrator、ChapterComposerOrchestrator、story_image_tasks、chapter_cover_tasks，图片生成从正文占位符改为结构化 intent -> asset -> markdown 回填。

3. 并发与一致性：为 story/chapter intent 增加 claim_token、claimed_at、attempt_count，采用数据库原子 claim 为主、Redis 锁为辅，避免重复生成、锁误删和 processing 卡死。

4. Memoir 读写路径：章节 canonical_markdown 成为正文真源，列表/详情接口补齐 markdown、cover_asset、word_count 等字段，PDF 与 asset 解析链路同步升级。

5. Memory / Retrieval：扩展 transcript ingest、chunking、evidence 检索与 story 聚合基础设施，为后续 story-first RAG 与多 agent 编排提供底座。

6. App 端体验：章节页继续走 MarkdownRenderer 阅读链，同时吸收 fix3-19 的跨平台 UI glitch 修复；更新对话页、首页、文案资源与章节列表映射逻辑。

7. 测试与文档：补充 asset resolver、story image task、章节封面派发、markdown 映射等回归测试，并加入图片占位符退役设计文档。

app-expo/src/app/(main)/conversation/[id].tsx

@@ -20,6 +20,7 @@ import {
   Platform,
   Pressable,
   StyleSheet,
+  Text as RNText,
   TextInput,
   View,
 } from 'react-native';
@@ -350,16 +351,32 @@ function VoiceRecordButton({
       ]}
       disabled={!enabled}
     >
-      <Text
-        style={[
-          styles.voiceRecordLabel,
-          !enabled && styles.voiceRecordLabelDisabled,
-          isRecording && styles.voiceRecordLabelRecording,
-        ]}
-        numberOfLines={1}
-      >
-        {isRecording ? tapToEndLabel : tapToStartLabel}
-      </Text>
+      {isRecording ? (
+        <View style={styles.voiceRecordLabelCenterWrap}>
+          <Text
+            style={[
+              styles.voiceRecordLabel,
+              styles.voiceRecordLabelCenter,
+              styles.voiceRecordLabelRecording,
+              !enabled && styles.voiceRecordLabelDisabled,
+            ]}
+            numberOfLines={1}
+          >
+            {tapToEndLabel}
+          </Text>
+        </View>
+      ) : (
+        <Text
+          style={[
+            styles.voiceRecordLabel,
+            !enabled && styles.voiceRecordLabelDisabled,
+            styles.voiceRecordLabelCenter,
+          ]}
+          numberOfLines={1}
+        >
+          {tapToStartLabel}
+        </Text>
+      )}
       {isRecording && (
         <View style={styles.voiceRecordPill}>
           <Animated.View
@@ -368,11 +385,16 @@ function VoiceRecordButton({
               { transform: [{ scale: pulseAnim }] },
             ]}
           />
-          {/* TODO: Duration number centering still broken on Android */}
           <View style={styles.voiceRecordDurationWrap}>
-            <Text style={styles.voiceRecordDuration}>
+            <RNText
+              style={[
+                styles.voiceRecordDuration,
+                Platform.OS === 'android' && styles.voiceRecordDurationAndroid,
+              ]}
+              {...(Platform.OS === 'android' && { includeFontPadding: false })}
+            >
               {formatRecordingDuration(recordingDuration)}
-            </Text>
+            </RNText>
           </View>
         </View>
       )}
@@ -505,7 +527,7 @@ function ChatInputBar({
           accessibilityRole="button"
         >
           <Text
-            style={[styles.sendButtonText, { color: colors.primaryForeground }]}
+            style={[styles.sendButtonText, { color: CHAT_COLORS.onSurface }]}
           >
             {sendLabel}
           </Text>
@@ -580,11 +602,21 @@ export default function ConversationScreen() {
   const [input, setInput] = useState('');
   const [inputMode, setInputMode] = useState<InputMode>('text');
   const [isKeyboardVisible, setIsKeyboardVisible] = useState(false);
+  const [keyboardHeight, setKeyboardHeight] = useState(0);
   const listRef = useRef<FlatList>(null);
 
   useEffect(() => {
-    const onShow = () => setIsKeyboardVisible(true);
-    const onHide = () => setIsKeyboardVisible(false);
+    const onShow = (e: { endCoordinates: { height: number } }) => {
+      setIsKeyboardVisible(true);
+      setKeyboardHeight(e.endCoordinates.height);
+      InteractionManager.runAfterInteractions(() => {
+        listRef.current?.scrollToEnd({ animated: true });
+      });
+    };
+    const onHide = () => {
+      setIsKeyboardVisible(false);
+      setKeyboardHeight(0);
+    };
     const subShow = Keyboard.addListener('keyboardDidShow', onShow);
     const subHide = Keyboard.addListener('keyboardDidHide', onHide);
     return () => {
@@ -621,11 +653,12 @@ export default function ConversationScreen() {
 
   const keyboardOffset = Platform.OS === 'ios' ? insets.top + 56 : 0;
   const kavEnabled = inputMode === 'text' && isKeyboardVisible;
+  const kavBehavior = Platform.OS === 'ios' ? 'padding' : 'height';
 
   return (
     <KeyboardAvoidingView
       style={styles.container}
-      behavior={kavEnabled ? 'padding' : undefined}
+      behavior={kavEnabled ? kavBehavior : undefined}
       keyboardVerticalOffset={keyboardOffset}
     >
       <View style={styles.column}>
@@ -668,8 +701,14 @@ export default function ConversationScreen() {
         <FlatList
           ref={listRef}
           style={styles.list}
-          contentContainerStyle={styles.listContent}
+          contentContainerStyle={[
+            styles.listContent,
+            inputMode === 'text' &&
+              isKeyboardVisible && { paddingBottom: 12 + keyboardHeight },
+          ]}
           data={flattenedData}
+          keyboardShouldPersistTaps="handled"
+          keyboardDismissMode="on-drag"
           keyExtractor={(item) => item.listKey}
           renderItem={({ item }) => (
             <MessageBubble
@@ -784,6 +823,7 @@ const styles = StyleSheet.create({
     flex: 1,
   },
   listContent: {
+    flexGrow: 1,
     paddingHorizontal: 12,
     paddingTop: 16,
     paddingBottom: 12,
@@ -952,6 +992,14 @@ const styles = StyleSheet.create({
     color: 'rgba(27, 27, 31, 0.72)',
     flex: 1,
   },
+  voiceRecordLabelCenter: {
+    textAlign: 'center',
+  },
+  voiceRecordLabelCenterWrap: {
+    flex: 1,
+    justifyContent: 'center',
+    alignItems: 'center',
+  },
   voiceRecordLabelRecording: {
     color: 'rgba(27, 27, 31, 0.92)',
   },
@@ -980,8 +1028,13 @@ const styles = StyleSheet.create({
   },
   voiceRecordDuration: {
     fontSize: 12,
+    lineHeight: 12,
     color: 'rgba(27, 27, 31, 0.86)',
+    textAlign: 'center',
   },
+  voiceRecordDurationAndroid: {
+    textAlignVertical: 'center',
+  } as const,
   textInput: {
     fontSize: 16,
     lineHeight: 22,