feat: refactor channel mapping with structured parameters and shared channel integration

- Add ChannelMappingRequest and ChannelMappingDeleteRequest structs with validation
- Update BridgeManager interface to accept structured parameters instead of individual strings
- Implement proper user ID and team ID propagation to shared channels
- Add shared channel creation/deletion integration with Mattermost API
- Update command handlers to provide user and team context
- Enhance logging with comprehensive parameter tracking

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/model/strings.go

@@ -0,0 +1,40 @@
+package model
+
+import "strings"
+
+// sanitizeShareName creates a valid ShareName matching the regex: ^[a-z0-9]+([a-z\-\_0-9]+|(__)?)[a-z0-9]*$
+func SanitizeShareName(name string) string {
+	// Convert to lowercase and replace spaces with hyphens
+	shareName := strings.ToLower(name)
+	shareName = strings.ReplaceAll(shareName, " ", "-")
+
+	// Remove any characters that aren't lowercase letters, numbers, hyphens, or underscores
+	var validShareName strings.Builder
+	for _, r := range shareName {
+		if (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') || r == '-' || r == '_' {
+			validShareName.WriteRune(r)
+		}
+	}
+
+	result := validShareName.String()
+	if result == "" {
+		return "matrixbridge" // fallback if no valid characters
+	}
+
+	// Ensure it starts with alphanumeric
+	for len(result) > 0 && (result[0] == '-' || result[0] == '_') {
+		result = result[1:]
+	}
+
+	// Ensure it ends with alphanumeric
+	for len(result) > 0 && (result[len(result)-1] == '-' || result[len(result)-1] == '_') {
+		result = result[:len(result)-1]
+	}
+
+	// Final fallback check
+	if result == "" {
+		return "matrixbridge"
+	}
+
+	return result
+}