Implement role detection service with pattern matching

- Created RoleDetector class with colon and bracket pattern extraction
- Implemented role deduplication logic
- Added fallback to default 'Narrator' role when no patterns found
- Created /api/tts/roles endpoint using RoleDetector
- Added comprehensive test suite for role detection
- All tests passing (7/7)

Requirements: 2.1, 2.2, 2.3, 2.4, 2.5, 2.6

backend/services/role_detector.py

@@ -0,0 +1,93 @@
+"""
+Role detection service for identifying distinct speakers in text.
+"""
+import re
+from typing import List, Set, Dict
+
+
+class RoleDetector:
+    """
+    Service for detecting distinct speaker roles in text.
+    
+    Supports two pattern types:
+    1. Colon pattern: "RoleName:" at the start of a line
+    2. Bracket pattern: "[RoleName]" preceding dialogue
+    """
+    
+    # Regex patterns for role detection
+    COLON_PATTERN = r'^([A-Z][a-zA-Z\s]+?):\s'
+    BRACKET_PATTERN = r'\[([A-Z][a-zA-Z\s]+?)\]'
+    DEFAULT_ROLE = "Narrator"
+    
+    def detect_roles(self, text: str) -> List[Dict[str, str]]:
+        """
+        Analyzes text to identify distinct speaker roles.
+        
+        Args:
+            text: Input text to analyze
+            
+        Returns:
+            List of role dictionaries with 'id' and 'label' fields.
+            Returns default "Narrator" role if no patterns found.
+        """
+        if not text or not text.strip():
+            return [{"id": self.DEFAULT_ROLE, "label": self.DEFAULT_ROLE}]
+        
+        # Extract roles from both patterns
+        colon_roles = self._extract_colon_pattern(text)
+        bracket_roles = self._extract_bracket_pattern(text)
+        
+        # Combine and deduplicate
+        all_roles = colon_roles.union(bracket_roles)
+        
+        # If no roles found, return default
+        if not all_roles:
+            return [{"id": self.DEFAULT_ROLE, "label": self.DEFAULT_ROLE}]
+        
+        # Convert to role dictionaries and sort for consistent ordering
+        role_list = [
+            {"id": role.strip(), "label": role.strip()}
+            for role in sorted(all_roles)
+        ]
+        
+        return role_list
+    
+    def _extract_colon_pattern(self, text: str) -> Set[str]:
+        """
+        Extracts roles from "RoleName:" pattern at line starts.
+        
+        Args:
+            text: Input text to analyze
+            
+        Returns:
+            Set of role names found
+        """
+        roles = set()
+        pattern = re.compile(self.COLON_PATTERN, re.MULTILINE)
+        
+        for match in pattern.finditer(text):
+            role_name = match.group(1).strip()
+            if role_name:
+                roles.add(role_name)
+        
+        return roles
+    
+    def _extract_bracket_pattern(self, text: str) -> Set[str]:
+        """
+        Extracts roles from "[RoleName]" pattern.
+        
+        Args:
+            text: Input text to analyze
+            
+        Returns:
+            Set of role names found
+        """
+        roles = set()
+        pattern = re.compile(self.BRACKET_PATTERN)
+        
+        for match in pattern.finditer(text):
+            role_name = match.group(1).strip()
+            if role_name:
+                roles.add(role_name)
+        
+        return roles