feat: 添加豆包 ASR 热词功能支持

- 在 config.yaml 中添加 hotwords 配置项，支持本地管理热词列表
- 实现热词解析、格式化和表名生成工具（internal/asr/hotwords.go）
- 在 ASR 连接建立时自动将热词发送给豆包（boosting_table_name 参数）
- 支持热词权重配置（1-10，默认 4），格式："词|权重" 或 "词"
- 支持配置热重载，修改热词后新连接自动生效
- 为未来动态热词功能预留扩展接口

热词格式示例：
  hotwords:
    - 张三|8
    - VoicePaste|10
    - 人工智能|6

internal/asr/hotwords.go

@@ -0,0 +1,107 @@
+package asr
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"strconv"
+	"strings"
+	"unicode/utf8"
+)
+
+const (
+	// DefaultHotwordWeight is the default weight when not specified.
+	DefaultHotwordWeight = 4
+	// MaxHotwordWeight is the maximum allowed weight.
+	MaxHotwordWeight = 10
+	// MinHotwordWeight is the minimum allowed weight.
+	MinHotwordWeight = 1
+	// MaxHotwordLength is the maximum character count per hotword.
+	MaxHotwordLength = 10
+)
+
+// HotwordEntry represents a single hotword with its weight.
+type HotwordEntry struct {
+	Word   string
+	Weight int // 1-10, default 4
+}
+
+// ParseHotwords parses raw hotword strings from config.
+// Format: "word|weight" or "word" (default weight 4).
+// Returns error if any hotword is invalid.
+func ParseHotwords(raw []string) ([]HotwordEntry, error) {
+	if len(raw) == 0 {
+		return nil, nil
+	}
+
+	entries := make([]HotwordEntry, 0, len(raw))
+	for i, line := range raw {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+
+		parts := strings.Split(line, "|")
+		word := strings.TrimSpace(parts[0])
+
+		// Validate word length
+		if utf8.RuneCountInString(word) > MaxHotwordLength {
+			return nil, fmt.Errorf("hotword %d: exceeds %d characters: %q", i+1, MaxHotwordLength, word)
+		}
+		if word == "" {
+			return nil, fmt.Errorf("hotword %d: empty word", i+1)
+		}
+
+		// Parse weight
+		weight := DefaultHotwordWeight
+		if len(parts) > 1 {
+			weightStr := strings.TrimSpace(parts[1])
+			if weightStr != "" {
+				w, err := strconv.Atoi(weightStr)
+				if err != nil {
+					return nil, fmt.Errorf("hotword %d: invalid weight %q: %w", i+1, weightStr, err)
+				}
+				if w < MinHotwordWeight || w > MaxHotwordWeight {
+					return nil, fmt.Errorf("hotword %d: weight %d out of range [%d, %d]", i+1, w, MinHotwordWeight, MaxHotwordWeight)
+				}
+				weight = w
+			}
+		}
+
+		entries = append(entries, HotwordEntry{
+			Word:   word,
+			Weight: weight,
+		})
+	}
+
+	return entries, nil
+}
+
+// FormatHotwordsTable generates Doubao-compatible hotword table content.
+// Format: each line "word|weight\n".
+func FormatHotwordsTable(entries []HotwordEntry) string {
+	if len(entries) == 0 {
+		return ""
+	}
+
+	var sb strings.Builder
+	for _, e := range entries {
+		sb.WriteString(e.Word)
+		sb.WriteString("|")
+		sb.WriteString(strconv.Itoa(e.Weight))
+		sb.WriteString("\n")
+	}
+	return sb.String()
+}
+
+// GenerateTableName generates a unique table name based on hotword content.
+// Uses SHA256 hash of the formatted table content (first 16 hex chars).
+func GenerateTableName(entries []HotwordEntry) string {
+	if len(entries) == 0 {
+		return ""
+	}
+
+	content := FormatHotwordsTable(entries)
+	hash := sha256.Sum256([]byte(content))
+	return "voicepaste_" + hex.EncodeToString(hash[:])[:16]
+}